Bi-2212 Grains Research Articles

Critical current density dependence of bulk textured Bi-2223 on thermal treatments and cold intermediate pressing

We report bulk Bi-2223 transport measurements for samples of various synthesis conditions. The measurements were carried out at 77 K in self-field and the critical current density, J C, reaches up to 12 000 A/cm 2 depending on the sample's preparation. The samples were prepared using a polymer matrix method. The same calcination procedure was performed on all the samples. After milling and pelletizing, calcined pellets were either directly sinter-forged (838 °C/20 h/10 Mpa) or sintered at 838 °C for various times (25–50–75–100–150–200 h) before being sinter-forged under the same conditions as the just-calcined pellets. The resulting hot-forged discs were analysed in terms of texture quality (50% of the Bi-2223 grains are aligned within an angle ranging from 6.2° to 3.9°) and Bi-2223 phase percentage (ranging from 26% to 95%). The discs were cut into bars which were either directly annealed (825 °C/100 h+1 °C per hour to 800 °C under 7.5% O 2 balance N 2) or subjected to various (1–3) sintering steps (838 °C/50 h) with or without intermediate cold pressing (1 GPa) before annealing. Transport measurements were performed on these samples, and the results are discussed in terms of Bi-2223 phase percentage, texture quality, grain boundaries, effect of cold intermediate pressing and post sintering. This novel type of bulk synthesis process is similar to the one used for tapes, and it allows the preparation of more reproducible bulk samples with high J C values and better grain connectivity.

Bi 2Sr 2CaCu 2O x bulk superconductor with MgO particles embedded

MgO nano-size particles in various concentrations were added to the Bi-2212 superconductor precursor before partial melting. The resulting ceramic superconductors had a transition sharpness and volume fraction which increased with MgO addition up to 9 wt.%. Compositional variations in the superconducting matrix with varying MgO addition were found, with possible important consequences for the pinning mechanism, since compositional variations are expected to be accompanied by variations in T c. By TEM it was directly documented that MgO particles could be embedded within the superconducting Bi-2212 grains. Hence wetting occurs. Thus, the requirements for MgO to play an effective role in pinning are satisfied. It is suggested that two distinct pinning mechanisms, both related to MgO addition, are likely to be present. Magneto-optic images clearly confirm the improvement of pinning, and screening, with increasing addition of MgO to Bi-2212.

Microstructural dynamics of Bi-2223/Ag tapes annealed in 8% O2

The microstructural dynamics of Bi-2223 tapes are investigated in situ during annealing in 8% O2 by means of 100 keV x-ray diffraction. A green mono- and a green multi-filamentary tape are annealed at 829.5 °C. During ramp-up (Ca,Sr)2PbO4 decomposes above 750 °C, resulting in an incorporation of Pb in Bi-2212. The associated grain growth of Bi-2212 is the main cause of the strain relief and the c-axis grain alignment of the Bi containing phases. Above 825 °C the Bi-2212 partly dissociates into (Ca,Sr)14Cu24Ox and a liquid phase. The linewidth of Bi-2212 is constant during the transformation to Bi-2223, indicating no strain or finite-size broadening. The most probable transformation mechanism is found to be nucleation and growth with a fast decomposition of the individual Bi-2212 grain, followed by a growth of Bi-2223 from the Bi-2212 melt reacting with (Ca,Sr)14Cu24Ox. The multi-filamentary tape transforms faster and exhibits a much better degree of c-axis alignment. Results are compared to similar studies in air. During a final annealing of a fully converted tape no change is found in the average concentration, stoichiometry and grain misalignment. A post-annealing experiment at 650 °C on quenched tapes shows a reaction over 3 weeks, whereby randomly oriented 3222 grains are formed and Bi-2223 is depleted of Pb. Simultaneously, the critical current decreases by a factor of 2.

Structural and Mechanical Properties of Particulate, Whisker and Unidirectional Bi-2223/Ag Composites

In order to obtain high Tc superconducting materials with improved mechanical properties, Bi1,5Pb0,5Sr2Ca2Cu3Ox / Ag-composites with 3 different filler morphologies were successfully synthesised. It is shown that Ag-addition improves Bi-2223 grain growth and reduces sample-porosity. An alignment of the ceramic phase along the surface of the Ag was observed and the results of TEM, SEM, DTA and EDX mapping are discussed, conclusions concerning the alignation-mechanisms based upon these observations are compared with popular models described in literature. Composites exhibit increased ductility and isostrain conditions are met, Little effect on the mechanical strength is observed but the mismatch in thermal expansion coefficient induces a residual stress field at the filler matrix interface and allows toughening mechanisms to appear, resulting in a clear post-peak behaviour. SEM pictures of whisker debonding, crack bridging and whisker pullout, all demonstrating a suitable filler-matrix interface, are included. The role of the filler morphology and the sequence of the processing steps on the toughening efficiency will be discussed.

Critical current densities of AgCu-sheathed Bi-2212 multifilamentary round wires in long length

Ag–Cu alloy sheathed Bi-2212 round wires have been fabricated by the powder-in-tube method. Short samples show critical current ( I c) values of more than 150 A at 4.2 K and 10 T, while I c for a small coil with total length of 10 m reduces to about 30% of the values obtained for short samples. In order to clarify the factors to deteriorate I c for coils, superconducting properties and the microstructures have been examined. The difference in transport I c between short samples and coils can be attributed not to the intrinsic properties of Bi-2212 grains but to the extrinsic limiting factors, such as voids that are formed by the released gases inside the wires with long length during heat treatment.

Optimization of sintering conditions on the Bi-2223 formation and grain size

Phases equilibrium between Bi-2201, Bi-2212 and Bi-2223 phases has been established depending on the sintering time and the sintering temperature. Starting with calcined powder (820°C/24 h), optimization of sintering conditions (temperature, time and intermediate milling) has been achieved. Three sintering steps are necessary to obtain high Bi-2223 phase content. The sintering conditions can be described as follows: 100 h at 850°C, first intermediate milling, 25 h at 850°C, second intermediate milling and 25 h at 835°C. The final Bi-2223 phase content reaches 93% with a resulting grain size between 2 and 3 μm.

Transmission electron microscopy investigation of Bi-2223/Ag tapes

The microstructure of (Bi,Pb) 2Sr 2Ca 2Cu 3O x (Bi-2223) tapes has been investigated by means of transmission electron microscopy (TEM) and high-resolution TEM. The emphasis has been placed on: (1) an examination of the grain morphology and size, (2) grain and colony boundary angles, which are formed during the tape processing, (3) a study of the grain boundaries on an atomic scale, including intergrowth investigations. Tapes with different process parameters have been compared with respect to the microstructure. A fully processed tape has on the average 50% thicker Bi-2223 grains than a tape after the first annealing. The angles of c-axis tilt grain boundaries are on average 14° and 26° for the fully processed tape and the tape after the first annealing, respectively. The intergrowth content (15%) and distribution are similar in these two tapes.

Evolution of core density of Ag-clad Bi-2223 tapes during process

The porosity and its effect on critical current density J/sub c/ of Ag-clad (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (Bi-2223) tapes throughout important steps in their thermomechanical treatment was investigated by using mass density measurement, microstructural observation, and superconducting property characterization. The relative mass density of the final filament for 19 filament tape was less than 75%, even though the J/sub c/ is /spl sim/50 kA/cm/sup 2/ (77 K, 0 T). The mass density in monofilaments reached 90%, because retrograde densification during the first heat treatment (HT1) was less or even absent. Well textured Bi-2223 grain growth could result in a significant densification during the first HT in the monofilament composite. Our results hint at considerable variability in this important property of Bi-2223 tape.

Formation and growth mechanisms of Bi-2212/Ag composite tapes

The microstructure changes during the solidification process of Bi-2212 tapes with several nominal compositions of Sr and Ca were investigated using an oil-quenching technique. The formation mechanism was almost the same in all composition tapes. Non-superconducting phases such as Bi-free and Cu-free solid phases exist in the liquid at T/sub max/. The quantity of the non-superconducting phases of Sr/Ca=1.8/1.2 and 2.2/0.8 tapes was larger than those of the 2.0/1.0 tape. With the solidification of the Bi-2212 phase near the oxide/silver interface, the non-superconducting phases melted into the liquid phase and shrank. Bi-2212 grains grew along the silver interfaces. The high J/sub c/ enhancement of the Bi-2212/Ag tape was explained by the excellent grain alignments at the oxide/silver interfaces. Locally, biaxially aligned Bi-2212 was observed at the oxide/silver interface for tapes fabricated using melt-solidification under a temperature gradient.

Effect of Various Intermediate Deformation Techniques on Jc and Grain Texture for Ag/Bi-2223 Tapes

Three Ag sheathed Bi-2223 multifilamentary tapes were produced by a processing method that consists of two sintering treatments with an intermediate deformation, involving sandwich rolling (SR), pressing (P), or normal rolling (NR). The magnetic field dependence of the critical current density Jc was measured with the magnetic field H applied parallel to both the ab plane (H ‖ ab) and the c-axis (H ‖ c) of the Bi-2223 grains. Experimental results show that Jc of the pressed (P) tape (JcP) for both H ‖ ab and H ‖ c is about 1.5–1.8 times higher than that for the NR tape (JcNR) and the SR tape, although JcSR is always larger than JcP. The ratio of JcSR/JcNR for H ‖ c increases rapidly with the applied magnetic field and reaches a maximum of about 12 at μ0H ≥ 900 T. The calculated density of the pinning force F as a function of magnetic field shows that curves of F for SR, NR, and P tapes all have their maximum Fmax at different magnetic fields and the magnitudes of Fmax are also different from each other. The SR tape has the largest value of Fmax, while NR has the smallest. XRD analysis shows that an intermediate deformation can destroy the grain alignment, and the larger the deformation, the worse the grain texture will be. Our experimental results, however, clearly show that Jc for Bi-2223 multifilamentary tapes is independent of grain alignment. The significant differences in Jc for tapes processed using the three different intermediate deformation procedures are dependent on the density of the pinning force and cannot be attributed to the grain alignment. Our experimental results support the view that SR processing is the best method for fabricating Ag/Bi-2223 tapes of high quality.

Bi2223超伝導相とSr-V-O酸化物の界面反応

We have developed Ag-sheathed multifilamentary Bi2223 ((BiPb)2Sr2Ca2Cu3Ox) tapes with Sr6V2O11(SVO) barriers. The superconducting properties of the critical current density, Jc and a. c. losses of the tapes strongly depend on the purity of SVO, especially on the residue of SrCO3 in it. In order to examine the behavior of an SVO barrier in detail, we prepared pellet samples with seven layers, which were stacked by barrier and Bi2223 layers one after the other, and examined the reaction between them. The five kinds of barrier layers, such as SrCO3, Sr6V2O11(6-2SVO), 5 and 10 mass%V2O5-added 6-2SVO, and Sr3V2O8 (3-2SVO), were used. The pellet samples were heat-treated at 1113 K for 72-360 ks in air. In the barriers of SrCO3 and 6-2SVO, including the residue of SrCO3, the large Bi2212 (Bi2Sr2Ca1Cu2Ox) grains are grown perpendicular to the interface to form the Bi2212 layers with a thickness of about 100 μm by the reaction between the barrier and Bi oxide. The majority of the Bi-oxide layers are composed of Bi2223 fine grains parallel to the interface. The thickness of the formed Bi2212 layers decreases when the content of SrCO3 in the barriers is decreased. In the reaction, SrCO3 reacts with the Bi oxides, and the elements of Pb and Cu diffuse and distribute homogeneously in the barrier layers. The residue of SrCO3 in 6-2SVO decreases obviously when the additional content of V2O5 is increased. In the 6-2SVO barriers with 5 and 10 mass% added-V2O5, only the very fine Pb oxide grains of (Sr, Ca)2PbO4 are precipitated in a row along the interface. No precipitation occurs for the 3-2SVO barrier. The 6-2SVO is stable, and the 3-2SVO is transformed into the 6-2SVO phase during the heat treatment. We can conclude that, for the use of 6-2SVO as a barrier, it is very important to remove the residue of SrCO3 in the 6-2SVO to prevent the formation of Bi2212 layers and, for that purpose, a small addition of V2O5 is effective.

Kinetics study of the Bi-2223 grain growth thickness

A kinetic study of the Bi-2223 phase formation mechanism has been done at temperatures ranging from 830°C to 865°C and for long sintering times (25–100 h), since the formation of Bi-2223 phase slows down. Above a sintering temperature of 850°C, the Bi-2223 decomposition begins to occur. The reaction order, n, depends on the sintering temperature; from 830°C to 850°C, n is roughly equal to 0.45 and the related mechanism corresponds to the thickening of plates after their edges have impinged. For temperatures higher than 850°C, n decreases to reach 0.13 at 865°C. Using the Johnson–Mehl–Avrami equation, an activation energy of 1437 kJ/mol has been found for temperatures between 835°C and 850°C where the partial melting becomes more significant. This activation energy has been shown to correspond to the thickening of the Bi-2223 grain.

Effect of nitrogen on supersolidus sintering of high speed steel

We have developed high performance silver sheathed Bi2223 wire. The progress in three years is especially remarkable. This dramatic development has been made with the controlled overpressure (CT-OP) sintering technique. The CT-OP sintering technique improves not only the critical current but also other performances such as n-value and mechanical strength, because of the improvement of the relative density up to almost 100%, the homogenization of the Bi2223 phase and the stronger connectivity between the Bi2223 grains. The dense Bi2223 wire prevents the ballooning phenomenon which is caused by the gasification of the trapped liquid nitrogen during warming up to room temperature.Present critical current has reached as about twice high as that in those days and shows over 200 A (77 K, self-field), which showed an excellent critical current property in a magnetic field.

Experimental study of the effect of filament orientation on transportand magnetic ac loss in Bi-2223/Ag multifilamentary tapes

Losses of electromagnetic energy in cyclic regimes were investigatedfor two tapes with flat Bi-2223 grains in different directions with respect tothe tape wide face. This was achieved through having different angles betweenthe wide faces of strip-like filaments and that of the tape: in the first tapethis angle was zero while in the second it was a right angle. In transport(self-field) conditions, comparison with theoretical predictions for ahomogeneous and isotropic material revealed opposite deviations at lowcurrents. The tape with parallel filament orientation exhibited higher lossthan that extrapolated from the critical current density measured in the fullysaturated state, and for the tape with transversal filaments it was lower thanexpected. We explain this through the different orientation of filaments withrespect to the self-field in the boundary shell carrying the ac current at lowamplitude. We also performed measurements in the external magnetic field,which was always perpendicular to the tape wide face. Magnetization resultscan be interpreted in terms of flux penetration in individual filaments.Spread in filament quality caused the absorption peak to be broader thanexpected for an ideal, homogeneous material.

Effect of Pb content variation on microstructure and critical current density of Bi-2223/Ag superconducting tapes

The effect of Pb content cariation on microstructure, phase evolution, Bi-2223 grain growth and critical current density (J c) of silver sheathed Bi-2223 tapes has been studied. The results indicate that high Pb content samples have higher conversion rate of Bi-2212 to Bi-2223 than low Pb content samples. However, too high Bi-2223 formation rate is not beneficial to Bi-2223 grain growth and thus the achieved critical current density in the tapes. The influence of coarse particles in precursor powder on the phase formation has also been investigated.

The changes of microstructure during solidification in Bi-2212/Ag tapes

We investigated the microstructure changes in Bi 2Sr 2CaCu 2O y /Ag composite tape during cooling from partially molten state by oil-quenching techniques. At maximum processing temperature T max, Bi-free phase and Cu-free phase exist in the liquid phase. The solidification of Bi-2212 phase starts near the free surface and oxide/silver interface. Bi-2212 grains grow along silver interface grow into the inside of molten oxide region randomly. No relation can be observed between Bi-2212 grain growth and grain boundaries of silver substrates.

AC loss measurement of Bi-2223 conductors

In this paper, AC losses of Bi-2223 tapes with different matrix of superconducting core were measured and analyzed. The two samples produced by a powder-in-tube method are multi-filamentary tape with Ag and AgAu matrix. The measurement was carried out under the environment of constant transport current and applied time-varying magnetic field. From experiment, losses for fields applied parallel to the c-axis of the textured Bi-2223 grains are larger by over an order of magnitude than those applied perpendicular. And it is confirmed that conductor losses with Ag matrix are larger than those with AgAu matrix.

Study of Ag/BSCCO interface in Ag-sheathed multifilament Bi-2223 tapes

High critical current densities ( J c ) in Ag-clad BSCCO tapes have been attributed to the presence of well aligned grains near the Ag sheath. Though the presence of Ag has been reported to aid in the alignment of Bi-2223 grains, the actual mechanism still remains unclear. In the present study effort was made to better understand the relationship between J c and grain alignment at the Ag/BSCCO interface. TEM studies were conducted on multifilament Bi-2223 tape as they a large Ag/BSCCO interfacial area. The interface region was examined to a distance of ≈2000 nm away from Ag sheath. In the high J c sample (≈20,000 A/cm 2), the kink angles between platelets were found be ⩽ 7°. For the low J c sample (≈2,000 A/cm 2), the kink angles were higher and ranged between 10–21° at the thickness of 200–500 nm. These results indicate that the supercurrent is strongly influenced by the alignment of BSCCO platelets near the silver interface.

Field angle dependence of the remanent magnetization of multifilamentaryAg/(Pb,Bi)2Sr2Ca2Cu3O10+xtapes: Microstructure and current carrying capability

We measured the dependence of the remanent magnetization of multifilamentary $(\mathrm{P}\mathrm{b},\mathrm{B}\mathrm{i}{)}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+x}$ tapes on the angle between tape surface and magnetic field. Remanent magnetization allows one to separate the intergrain and intragrain contributions. The angle dependence of the intergrain magnetization is accurately described by a thin strip model. The intragrain magnetization can be reproduced by regarding the microstructure as an agglomeration of plateletlike grains. The orientation of the grains is described by a Gaussian distribution. Based on this model we develop a comparatively simple method to determine the mean grain alignment within the tapes. A comparison of the grain alignment inferred from magnetization as well as transport measurements provides direct evidence for the presence of a critical angle for Bi-2223 grain boundaries.

Superconducting joints of Bi-2212/Ag tapes and laminates

Different types of HTS joints of Bi-2212/Ag tapes and laminates, which arefabricated by dip-coating and partial-melt processes, have been investigated.All joints are prepared using green single and laminated tapes and accordingto the scheme: coating-joining-processing. The heat treated tapes havecritical current (Ic) between 7 and 27 A, depending on tape thickness andthe number of Bi-2212 ceramic layers in laminated tapes. It is found that thecurrent transport properties of joints depend on the type of laminate, jointconfiguration and joint treatment. Ic losses in joints of Bi-2212 tapes andlaminates are attributed to defects in their structure, such as pores,secondary phases and misalignment of Bi-2212 grains near the Ag edges. Byoptimizing joint configuration, current transmission up to 100% is achievedfor both single tapes and laminated tapes.