热压处理对室温轧制Ag/Bi(2223)超导带材显微组织的影响

采用SEM和TEM观测了经热压处理后的Ag/Bi(2223) 带材(Jc＝56000A/cm2,在77K、自场)和只经过室温轧制处理的带材.与室温轧制的带材比较发现:即使在临近第二相的区域,热压处理后的带材结构也非常密实;热压处理后,晶体缺陷诸如裂缝、亚晶界、残余无定形态和中间相等,易阻碍或中断电流传输的缺陷显著减少;热压处理后带材的位错密度基本相同,但在某些区域分布不均匀或呈网状结构;在一定方向上晶界能捕获途经它的位错;热压过程促使第二相部分转变为超导相Bi(2223),进而提高带材的载流能力.     

Development of high-Tc superconducting Bi-2223 tape joints

High-T _c superconducting joints between Ag-clad Bi-2223 tapes have been developed for persistent current applications. Two presintered tapes with one side of the silver stripped were lapped and then wrapped by a silver foil. The complex was uniaxially pressed followed by appropriate sintering to form a high-T _c superconducting tape joint. It was found that the ratio of critical currents through the joint to that of the tape,I _cj/I_c, depended on the uniaxial pressure and the sintering conduction. At liquid-nitrogen temperature 77 K,I _cj/I_c=99% has been achieved. Persistent current loops formed by Bi-2223 tapes have also been fabricated and tested. Joint resistance of a loop was determined to be 4×10^–13 between the decay time of 120 and 3600 sec.     

Phase and Texture Evolution of Ag/Ni Composite-Sheathed Bi-2223 Tapes with Different Thickness

The influence of green tape thickness on the Bi-2223 phase formation and texture evolution in Ag/Ni composite-sheathed tapes fabricated by the “powder-in-tube” technique has been studied. Microstructural observations by SEM as well as critical current density (J _c) measurements at 77 K, 0 T have been performed to analyze the performance of the tapes. The results show an important influence of the green tape thickness on the critical current depending on the content and texture of Bi-2223 phase. The J _c increases with decreasing thickness. Moreover, texture measured by omega scans shows that the texture of the Bi-2223 phase is significantly influenced by the thickness of the green tape after the first and final sintering processes. Alignment of Bi-2223 grains in the thin tapes is much better. Higher performance of Ag/Ni composite-sheathed Bi-2223 tapes can be obtained by controlling the thickness of the green tapes.     

The influence of the first heat-treatment on the critical current density of (Bi,Pb)-2223/Ag superconducting tapes

Optimization heat-treatments have been performed on multi-filament Bi-2223/Ag superconducting tapes under 1 bar total gas pressure, the oxygen partial pressure being 8.5%. In a first heat-treatment (HT1), the tapes were sintered within 822-838 °C for 1-50 h. After intermediate deformation, all the samples underwent the second heat-treatment (HT2) at 825 °C and 830 °C for 20 h followed by a thermal sliding procedure. The relative content of the phases present in HT1 samples was measured by XRD. It was found that the Bi-2212 phase content after HT1 strongly influences the values of J_c after HT2. There is a correlation between the amount of Bi-2212 phase after HT1 and the final J_c values after HT2. A maximum of J_c was found for a ratio of 0.15 between Bi-2212 and Bi-2223.     

Filter cake forming and hot isostatic pressing for TZP-dispersed hydroxyapatite composite

The combination of a filter cake forming process and hot isostatic pressing was applied to prepare hydroxyapatite composites containing dispersed tetragonal zirconia polycrystal (TZP) with high strength and toughness. Fine TZP powder was dispersed into as-synthesized hydroxyapatite slurry, formed with the filter cake process and hot isostatically pressed at 800–1150 °C at 100 MPa for 2 h. The temperature needed for densification increased with increasing TZP content; 1100 °C was needed to fully densify the composite with 26.8 wt% TZP. No phase change was found in TZP nor in the hydroxyapatite phase up to the maximum temperature examined in hot isostatic pressing. Significant phase change was found in specimens annealed in air at 1200 °C. The strength and toughness achieved were respectively 190 MPa and 2.3 MPa m^1/2. These values were approximately 20% and 100% higher than the corresponding values for hydroxyapatite ceramics without TZP particle dispersion.     

Development of Bi-2223 HTS tape and its application to coil and current leads

We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I_c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.     

Microstructure and Superconducting Properties of TaC-Doped Bi-2223 Ceramics

The microstructure, phase composition, and superconducting properties of Bi-2223 ceramics doped with 0.05–0.5 wt % TaC were investigated. The materials were heat-treated at 840°C in three steps (10 + 24 + 24 h). By optimizing the TaC content and heat-treatment conditions, the 77-K critical current density of the Bi-2223 ceramics in zero field was raised by a factor of 2.     

Fabrication and Properties of Ag–Ni Bimetallic Sheathed (Bi,Pb)-2223 Tapes

10-meter-long Ag?CNi bimetallic sheathed (Bi,Pb)-2223 tapes with outer nickel sheath and inner silver sheath have been successfully fabricated by the ??Powder in tube?? technique. Microstructure and phase evolution studies by means of SEM and XRD, as well as critical current density (J _c ) measurements have been performed. It is found that the nickel sheath and dwell time in the first sintering process have great influences on the texture evolution, phase transformation and J _c of the Bi-2223/Ag/Ni tapes. Mono-filament (Bi,Pb)-2223 tape with a J _c of 6656?A?cm^?2 and 61-filament tape with a J _c of 12420?A?cm^?2 are obtained. Although using composite bimetallic sheaths can reduce production costs and improve mechanical properties of the Bi-2223 tapes, the Bi-2223 content and J _c of Bi-2223/Ag/Ni tapes are relatively lower than that of traditional Bi-2223/Ag tapes. Meanwhile, due to higher Bi-2223 content and better alignment of Bi-2223 grains, tapes with 61-filament have higher J _c than mono-filament tapes.     

Effects of Rolling Passes on the Transport Properties of 37-Filamentary AgAu-Sheathed Bi-2223 Tapes

Thirty-seven-filamentary AgAu-sheathed Bi-2223 tapes were fabricated by a powder-in-tube (PIT) process. And, the round wires (? 1.86 mm) were rolled to 0.35-mm tapes with 12, 7, 5, and 4 rolling passes through flat rolling, respectively. The influences of different rolling passes on the core density, deformation, and transport properties of Bi-2223/AgAu tapes were systematically investigated. It was noticed that after rolling, the Vickers microhardness of the superconducting core and deform homogeneity along both the horizontal and vertical directions on the cross section of seven-pass rolled tape were better than those on the tapes with other passes, which proved the larger core density and uniform deformation with the seven-pass rolling process. Meanwhile for the wires with 12 and 7 passes, the AgAu/superconducting core interfaces were much flatter. With the rolling passes decreasing from 12 to 4, the critical current density (J_c) first increased and then decreased. Due to the better homogeneity and flatter interfaces, J_c reached the maximum value of 17.3 kA/cm² on the seven-pass sample. Meanwhile, the enhancement of current capacities in magnetic field applied parallel to the Bi-2223/AgAu tape surface could also be recognized as the evidence of improving intergrain connections due to the higher density in seven-pass rolled tapes.     

Solid state synthesis and properties of monoclinic celsian

Monoclinic celsian of Ba0.75Sr0.25Al2Si2O8 (BSAS-1) and Ba0.85Sr0.15Al2Si2O8 (BSAS-2) compositions have been synthesized from metal carbonates and oxides by solid state reaction. A mixture of BaCO3, SrCO3, Al2O3, and SiO2 powders was precalcined at 900–940 °C to decompose the carbonates followed by hot pressing at 1300 °C. The hot pressed BSAS-1 material was almost fully dense and contained the monoclinic celsian phase, with complete absence of the undesirable hexacelsian as indicated by X-ray diffraction. In contrast, a small fraction of hexacelsian was still present in hot pressed BSAS-2. However, on further heat treatment at 1200 °C for 24 h, the hexacelsian phase was completely eliminated. The average linear thermal expansion coefficients of BSAS-1 and BSAS-2 compositions, having the monoclinic celsian phase, were measured to be 5.28 × 10–6 °C–1 and 5.15 × 10–6 °C–1, respectively, from room temperature to 1200°C. The hot-pressed BSAS-1 celsian showed room temperature flexural strength of 131 MPa, elastic modulus of 96 GPa and was stable in air up to temperatures as high as 1500 °C.     

The influence of electroplastic rolling on the mechanical deformation and phase evolution of Bi-2223/Ag tapes

Electroplastic rolling (EPR) of Bi-2223/Ag superconducting wires was performed, where pulse currents were applied during rolling to introduce an electroplastic effect. It was found that the rolling force decreased significantly compared with the traditional rolling process. Furthermore, EPR favorably minimized the sausage effect. It is revealed that the electroplastic effect can facilitate the mechanical deformation of Bi-2223/Ag composites. Segments of the Bi-2223/Ag tapes were heat treated at 830 °C for different time periods. The phase assemblies of these samples suggest that current pulses contribute to faster transformation kinetics from the Bi-2212 phase to the Bi-2223 phase. In addition, a preliminary improvement of 28% of critical current density has been achieved in a fully processed tape with EPR.     

Effect of magnetic field, thermal cycling and bending strain on critical current density of multifilamentary Bi-2223/Ag tape and fabrication of a pancake coil

Multifilamentary HTSC tapes are important for their applications in various electrical devices. Powder-in-tube technique with improved optimized synthesis parameters is regarded as one of the most promising ways to prepare long-length multifilamentary Bi-2223/Ag tapes. Nevertheless, usefulness of such tapes depends on their electrical and mechanical properties. Critical current density of a Bi-2223/Ag tape with 37 filaments has been studied at 77 K with field, field orientation, thermal cycling and bending strain as parameters. Results have been discussed in light of various mechanisms and models. A small pancake coil has been fabricated out of the same tape and the test results presented.     

Pressing Processing Diminishes Outgrowth and Bridging for AgMg/Bi-2223 Tapes

Multifilamentary Bi-2223 tapes were fabricated by PIT, using a silver alloy sheath with 2.5% magnesium metal. Outgrowth and bridging are major disadvantages for some silver alloy sheathed Bi-2223 tapes. To solve the outgrowth problem, green tapes were pressed with different reduction rates and then sintered at high temperatures. Critical current I _c of the sintered tapes was measured at nitrogen temperature using the four-probe method. The morphology of the filament core was observed with scanning electron microscope (SEM) to investigate outgrowth of sintered tapes with different reduction rates. Outgrowth and bridging on cross and longitudinal sections of filaments were studied using TEM images. Experimental results suggest that pressing processing may diminish outgrowth and bridging. Number of outgrowths and cases of bridging are reduced when the reduction rate increases, but the slope of the reduction falls at large reduction rates. The I _c curve indicates that there is an optimum reduction rate at which I _c reaches a maximum. At the best reduction rate the amount of outgrowth and bridging is close to the lowest. Experimental results show that pressing processing can diminish outgrowth and bridging by as much as 50%. Therefore, proper pressing is an effective method for both diminishing outgrowth and bridging and enhancement of I _c.     

Transport critical current density of Ag-sheathed Pr-doped (Tl,Cr)Sr2CaCu2O7 superconductor tapes in applied magnetic field

Powder-in-tube Tl-1212 (Tl,Cr)-Sr-(Ca,Pr)-Cu-O/Ag tapes were prepared and processed with different thermomechanical treatments. The processing conditions caused the variation of the transport critical current density (J_c) in the tapes. Tapes subjected to rolling showed lower J_c compared to the pressed tapes. Uniaxial pressing is more effective in densifying the tape cores by forcing the grains into closer contact and enhances the connectivity between grains. All the tapes showed a majority phase of 1212. The existence of 1201 phase considerably reduced J_c in the rolled tapes. Intermediate rolling is not favorable in improving J_c for the irregular grains structure, in contrast to Bi-2223 superconductors with plate-like grains. The in-field behavior of the tapes indicates that they are dominated by weak links as J_c falls rapidly in low applied field (H<0.06 T). No anisotropic transport properties were observed for all the tapes. SEM images showed randomly oriented irregular grains making texturing difficult to achieve. Neither the grain size nor the morphology was changed significantly by the thermomechanical treatments. The observed irregular grains imply that high-angle tilt boundaries were formed with intergranular weak links. Grain morphology is suggested to be the key factor that limits the J_c in these tapes.     

Tape Casting of Al_2O_3-TiB_2/Al_2O_3-Nano-TiC Multilayer Composites

Ceramic tapes, containing Al2O3-25 wt pct TiB2(B) and Al2O3-25 wt pct nano-TiC (c), have been obtained by tape casting process. Numerous tapes (about 60～80 tapes) were prepared by stacking in turn the composition (B) and (C), laminating under 10 MPa pressure, eliminating the solvent and burning out the polymer additives. The final green bodies were hot pressed at 1750℃ and 30 MPa. The composite has a bending strength of 568 MPa and a fracture toughness of 5.8 M Pa·m1/2. SEM analysis exhibits that Al2O3 particle growth was inhibited by TiC particles in C. but TiB2 particles could not hinder Al2O3 growth in B. The curves of GTA indicates that all organic additives could be removed completely above 600℃     

The J c Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

Hot pressing of SiC-TiC composites

Sintering behaviour during hot pressing of SiC-TiC composite ceramics has been investigated with special emphasis on the effect of various processing parameters on the density and mechanical properties of the sintered body. At hot pressing temperatures greater than 2000° C, significant densification occurred in SiC-50 wt%TiC (–0.5 wt%B-1 wt%C) composites. The room temperature flexural strength of the sintered body increased with the hot pressing temperature up to 2000° C and reached a highest value of 710 MPa in accordance to the variation of density with temperature. In sintering of composites without additives, densification was enhanced with the addition of up to 25 wt%TiC, with relative densities higher than 98% observed when hot pressing at 2150° C for 2h.     

The Jc Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

Diphasic mullite gel pressed at 1.5 GPa: Mechanical properties and microstructure of ceramics

Mullite gels containing 72–85 wt% Al2O3 were prepared by mixing commercial boehmite and silica sols and gelled by heating. Powdered gels were either dried at 60°C or calcined at 550°C, and cold isostatically pressed under 0.5, 1.0 or 1.5 GPa. The effect of pressure is greater for the calcined gels than for the dried gels. The bulk density and room-temperature flexural strength of ceramics increased with increasing compacting pressure, while the fracture toughness is independent of the pressure. Mullite containing 75 wt% Al2O3 and sintered at 1500°C for 3 h has a density which is about 99% of the theoretical density, a flexural strength of 523±40 MPa and a fracture toughness of 3.5 MPa m1/2.     

Fabrication and transport properties of long–length doped Bi1.7Pb0.3Sr2Ca2-xMxCu3Oy conductors (M=B, Nb or Ag)

Long lengths of Bi-2223 conductors doped with Ag, Nb and B were fabricated. The recrystallization of the glasses during various heat treatments was studied by differential thermal analysis (DTA), differential scanning calorimetry, X-ray diffraction and resistivity measurements. Activation energies and frequency factors for crystallization were determined by non-isothermal DTA, employing different models. It was found that both the peritectic transition and the reaction rate were dependent on the ambient atmosphere. Kinetic studies under different atmospheres revealed that the thermal stability of Bi-2223 was greatly enhanced under an oxygen atmosphere. Jc measurements at 77 K showed an increase on silver addition. Furthermore it was found that the silver addition does not destroy the superconductivity. © 1998 Chapman & Hall