Processing and transport properties of high-Jc silver-clad Bi-2223 tapes and coils

The powder-in-tube process has been used to fabricate long lengths of flexible, high-J_c, silver-clad Bi-2223 HTS conductors. By improving thermomechanical processing and precursor powder preparation, we have succeeded in achieving J_c values of≥4×10⁴ A/cm² at liquid nitrogen (77K) temperature and >10⁵ A/cm² at liquid helium (4.2K) and liquid neon (27K) temperatures in short tape samples. Detailed measurements with high applied magnetic fields are reported. Several long tapes up to 10 m in length have also been fabricated and cowound into small superconducting pancake coils by the “wind-and react” approach. Transport measurements at 77 and 4.2K for these coils are also reported.     

Sandwich Roiling,Jc, and pinning energy in Ag-sheathed BPSCCO superconducting tapes

A “sandwich rolling” process was developed to prevent the formation of sausaging and cracks in the longitudinal direction. The stress-strain state of the tape in “sandwich” rolling is the same as that of uniaxial pressed tape because the deformation of steel sheets is negligible in comparison to that of Ag-clad tape. Critical current densities of 3.2 × 10⁴ A/cm² at 77K and 2.7 × 105 A/cm² at 4.2K and zero field Ag-sheathed Bi-based 2212 tapes have been achieved using a melt and atmosphere-controlled process. The comparison of pinning potential U₀(B) < U(T-0, B) for Bi-2212 tape and Bi-2223 tapes consisting of a different fraction of 2212 phase as well as Bi-2212 and Bi-2223 thin films shows that for the same fields, the U_o for good quality 2223 tapes is at least 1.3 times that for the best 2212 tape and epitaxial thin films after taking into account the difference of the T_c between 2223 tape and 2212 tape, indicating that in BSCCO compounds, in addition to anisotropy, the specific pinning centers such as dislocations, introduced during processing, affect the flux motion at lower B.     

Effect of mechanical deformation on the mass density of Ag-clad (Bi,Pb)2Sr2Ca2Cu3O10 wire and tape

The mass densities of Ag-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ wire and tape varies during the mechanical deformation process, which is one of the steps of the oxide-powder-in-tube technique used to fabricate the composite superconductor. Results show that the rolling has a more significant effect on densifying the tape core, whereas the drawing process can only densify the core to about 75% of the theoretical density. SEM observations of the rolled samples also reveal very dense morphology, consistent with the mass density calculations. SEM observation also shows that with increasing the deformation extent, the average grain size is reduced. It is proposed that although the rolling densifies the tape core significantly, it also destroys the crystallinity of the superconducting phases and results in the formation of an amorphous phase. Since the textured Bi-2223 phase forms by the epitaxial growth on the textured Bi-2212 seed crystals, the deformation induced texture is critical. Appropriate deformation extent is necessary, since too high an extent of deformation may change the well-aligned grains into amorphous phase. The formation of the amorphous phase is harmful to the texturing formation of the Bi-2223 phase, which finally leads to critical current degrading.     

Melt texturing of YBCO for high current applications

In recent years significant progress has been made in J_c enhancement in high T_c superconductors using melt texturing techniques. Among the many melt texturing methods and modifications, seeding and directional solidification techniques offer extensive control over the location of the growth front and the growth direction over long distances during melt texturing which makes these techniques most attractive from the standpoint of long conductor fabrication. These processes have the capability of producing J_cs of about 45 000 A cm⁻² across single domains of YBCO. A novel variant of the conventional melt texturing process called the liquid phase removal method provides a means to improve the grain boundary coupling in melt textured bulk polycrystalline HTS. Grain boundaries in samples processed by this technique with misorientation angles as high as 54° have demonstrated J_cs as high as 18 000 A cm⁻². Recent developments in texturing of RE-123 compounds (Nd and Yb) at high growth rates give promise for considerable reduction in processing times in directional solidification. Texturing has been observed even in samples processed at rates as high as 100 mm h⁻¹. With these advances in melt texturing methods, utilization of bulk HTS in practical applications such as high capacity current leads etc., appears to be a distinct possibility of the near future.     

First time in-situ investigation of 2223 formation measured on (Bi,Pb)2Sr2Ca2Cu3O10+x thick films on Ag tape prepared by the screen printing technique

Detailed investigations were performed on the reaction kinetics of the superconducting BPSCCO 2223 phase in order to be able to improve tape processing and critical current density J_c of 2223 layers on Ag tape made by the organic binder method. The J_c values presently vary between 6–12 × 10³A/cm² (77 K, O T). To our knowledge, it is the first time that the formation of the 2223 has directly been observed employing in-situ high temperature X-ray diffraction (HTXRD). This measurement was carried out on screen printed samples of BPSCCO on Ag foil heated in a specially designed closed crucible preventing the evaporation of lead. The main findings are that the 2223 phase arises from the 2212 phase at 820–835°C within a short timescale of about 2–3 h. These results were correlated with measurements on a Simultaneous Thermal Analysis (STA) showing endothermic reactions which indicate the starting of partial melting at about 825–836°C. According to the literature we found that the reaction kinetics are controlled by diffusion processes under the presence of partial melt.     

Rapid synthesis of Y- and Bi-based high-Tc cuprates by microwave irradiation

High-T_c superconducting samples of YBa₂Cu₃O_7−x with T_c∼90 K and Bi_2.2Sr_1.8Cu_1.05O_x with T_c∼9 K have been prepared for several ten min, using a domestic microwave oven operated at 2.45 GHz, without any post-heat-treatment. Post-heat-treatment is not necessary due to improvement of the sample environment during the microwave process. That is, a pellet of mixed powder of starting materials is surrounded by mixed powder of starting materials and, subsequently, wrapped in glass wool in order to suppress rapid dissipation of heat from the surface of the pellet. This leads to successful preparation of homogeneous and oxygenated samples. In addition, we have attempted to prepare Bi-2212 and Bi-2223 phases. A sample whose major phase was Bi-2212 was obtained. However, no sample with the Bi-2223 phase could be obtained.     

Effect of lead content on Bi-2223 formation by a two-powder process

Decreasing the total lead content from Pb_0.4 to Pb_0.3 significantly stunts Bi-2223 phase development and greatly reduces the critical current density (J_c) of powder-in-tube tapes made by a two-powder process. This effect can be explained on the basis of the solubility limit for lead in 2212. Pb_0.4 samples exceed the solubility limit for lead in 2212, so lead is rejected to create a lead-rich liquid that enhances the kinetics of 2223 formation during heat treatment. By contrast, a liquid does not form in the Pb_0.3 sample because its lead content is below the solubility limit. As a result, 2223 formation is much slower and J_c is much lower in the Pb_0.3 sample.     

Recent status on high temperature superconducting Bi2Sr2CaCu2O8+x wire development at NYSIS: 1-90 meter length Jcs and 3 meter diameter ring furnace design

The status of long length, Bi₂Sr₂CaCu₂O_8+x (Bi-2212) wire development at the New York State Institute on Superconductivity (NYSIS) is reviewed and updated. Transport J_cs (4.2K, 0 T) of Bi-2212/Ag oxide powder-in-tube singlefilamentary tapes have reached 70,000-80,000, 50,000-60,000, and 30,000–40,000 A/cm² for 1, 4–15, and 40–90 meter length tapes, respectively. The decrease in J_c as the tape length was increased from 15 to 90 meters was attributed to the (measured) sensitivity of J_c to temperature nonuniformities (±3‡C) in the box-type furnace used for annealing. To reduce this problem, a ringtype high-temperature furnace (∼3 meter diameter) was designed and constructed which provides a large-volume (∼13^w × 10^h × 1000¹ cm) processing zone with expected excellent temperature uniformity (±0.5‡C). The advantages of the ring-type furnace for processing of kilometer-length conductors are described.     

Fabrication of long length Bi-2223 superconductor tape using continuous electrophoretic deposition on round and flat substrates

We have developed a continuous fabrication process for producing long lengths of Bi-2223 superconductor tapes. The process involves sequentially electrophoretically depositing and sintering superconductor and then silver layers on a substrate, followed by rolling and thermal processing. Both round and flat silver substrates have been used. Bi-2223 tapes made using flat silver substrates require only a few processing steps. Transport critical current densities at 77K in zero applied magnetic field exceeding 20000 A/cm² have been obtained.     

On the manufacture of silver-sheathed Bi-2212 high temperature superconducting wires

High temperature superconductors (HTS) are presently being considered for a variety of defense and commercial applications. However, problems associated with low critical current density (J_c) in long length conductors still need to be addressed. It is realized that success in the fabrication of long lengths of wire and tape (up to 1 km) with optimum superconducting properties relies on improved manufacturing technologies. Unfortunately, there is no systematic study concerning the effect of processing parameters on the green properties of wires and tapes during the deformation processes. The focus of this investigation centered on silver (Ag) sheathed Bi-2212 wire forming processes and use of the two most common techniques, wire drawing and hydrostatic extrusion. Billets of 6.35 mm outside diameter were reduced separately by five different die sequences to a final diameter of 1.63 mm. Wires made by these procedures were characterized for properties such as Bi-2212 relative packing density, sheath material thickness, deformation force, elongation, drawing friction, and microstructural and mechanical properties. These results indicated that the amount of reduction per pass has a significant impact on the characteristics of non-heat treated wires and on the mechanical aspects of the wire drawing operations.     

Long lengths of silver-clad Bi2223 superconducting tapes with high current-carrying capacity

Long lengths of silver-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) high-T_c multifilamentary tapes were produced using the powder-in-tube (PIT) technique followed by a thermomechanical process. The relationships between microstructure and electrical, magnetic and mechanical properties of the heat treated tape were evaluated from the critical current density measurements, irreversibility magnetic field determination and mechanical bending tests. Emphasis was stressed on the J_c behavior in magnetic fields at different temperatures. A J_c of 10,000 A/cm² at 77 K in a zero field for a 10 m tape and 75,000 A/cm² at 23 K in a field of 3 T for a short tape was achieved. The results obtained showed that Bi2223/Ag high-T_c composite tapes are a potential alternative to conventional low-T_c superconductors in magnetic levitation (MAGLEV) applications.     

Bi-2223/Ag-sheathed tape processing studies

The production of high critical current density J_c Bi-2223/Ag sheathed conductors is a complex process involving interactions among many different parameters. The effects of three factors: 1) powder production path, 2) the first sinter temperature, and 3) the subsequent sinter temperatures were investigated. Statistical methods were used to design the experiment and interpret the results. Transport J_c was the main response for the analysis, but microstructural results were also used to assess the physical basis for the differences in performance. The powder variable had the largest main effect with only very weak main effects for the other factors.     

Fabrication and characteristics of tapes and test magnets made from Ag-Clad Bi-2223 superconductors

Pb_0.4Bi_1.8Sr₂Ca_2.2Cu₃O_x (Bi-2223) precursor powder was prepared by a solid-state reaction of carbonates and oxides of lead, bismuth, strontium, calcium, and copper, and the powder was then used to fabricate silver-clad tapes by the powder-in-tube technique. Transport critical current density (J_c) values>4×10⁴ A/cm² at 77K and 2×10⁵ A/cm² at 4.2 and 27K have been achieved in short tape samples. Long lengths of tape were tested by winding them into pancake coils. Recently, we fabricated a test magnet by stacking ten pancake coils, each containing three 16m lengths of rolled tape, and tested it at 4.2, 27 and 77K. A maximum generated field of 2.6 T was measured in zero applied field at 4.2K and the test magnet generated significant self-field in background fields up to 20 T. The results are discussed in this paper.     

Current-voltage characteristics and transport current distributions in Ag/Bi-2223 monocore tapes

Current-voltage characteristics of Ag-sheathed Bi-2223 monocore tapes have been measured at 77 K in low d.c. magnetic fields. Results have been obtained with the c-axis of the sample both parallel and perpendicular to the applied field, together with the force-free orientation. The current–voltage characteristics have been analysed in terms of their n-value (V∝Iⁿ), which characterizes the sharpness of the resistive transition and the second differential (d²V/dI²), which shows the distribution in local critical current density values. The second differential has been fitted and analysed using the phenomenological Weibull distribution function, which is based on a weak-link model. The four parameters of this function have been used to investigate the microstructural nature of the sample, in particular, the factors influencing the critical current density and the transport current pathways. The results are consistent with the view that at low fields the critical current density is limited by weak-link grain boundaries and at high fields by intra-granular flux pinning.     

Microstructures and superconducting properties of V doped Bi2223 tapes

We prepared Ag-sheathed tapes by the powder-in-tube method using V doped Bi2223 powder. We found that V doping into Bi oxide superconductors greatly enhances the Bi2223 phase formation of the tapes without any degradation of the superconducting critical temperature T_c. The doped V is not included in the Bi2223 (or Bi2212) grains, but instead exists as Sr₆V₂O₁₁ particles with diameters of 0.1 to several micrometers, in which the larger plate-like particles play an important role to promote the formation of the Bi2223 phase. With V doping, the optimum critical current density J_c value is obtained for a short time heat treatment.     

Fabrication of Ag-Sheathed Bi-2223 tapes using powders produced by aerosol spray pyrolysis

Bi-2223 tapes were manufactured from a fine “two-powder” product produced by using an aerosol spray pyrolysis technique. Critical current density of 22000 A/ cm2 at 77K and 0 T was achieved. Nondestructive transmission x-ray diffraction study indicated good alignment of the superconducting grains. The texturing process of the superconducting phase was found to be nearly complete after the first 24 h of heat treatment for the samples studied. Pressing was found to play little role in the texturing process. The texturing can be enhanced by Ag-doping. J_c, however, was not found to be improved significantly, presumably due to the reduced effective cross-sectional area. A new phase, Bi-4435, was identified which may play a significant role in the formation of 2223. On leave from Northeastern University, Shenyang, P.R.C. On leave from Kobe Steel Ltd., Kobe, Japan     

Structural and magnetic properties of silver doped melt-textured YBCO prepared in a solar furnace

Magnetic measurements and structural investigation have been performed on melt-textured YBCO and AgYBCO HTS. The “sun” technique produces very dense YBCO (ρ=5.86 g cm⁻³) and AgYBCO [ρ=6.36 g cm⁻³; 10% b/w (by weight) silver. This technique renders samples with a large volume fraction of the Y₂BaCuO₅ (211) phase. The material is characterized by very high J_c values, as compared with bulk polycrystalline YBCO prepared by other methods. This feature is attributed to the enhanced amount of 211 particles which serve as pinning centers. Additional significant densification of the structure due to silver incorporation is obtained, and a reduction of the size of 211 inclusions is also observed. Silver doped samples show “butterfly”-like hysteresis loops at relatively high temperatures (T≥60K). This feature is probably associated with oxygen deficiency which arises from the slower oxygen diffusion into silver doped samples. J_c values enhancement was obtained in silver doped “sun” samples at high temperatures (T≥60K) and fields of 20–30 kOe. The temperature dependence of effective activation energy of pinning, U_eff, was measured for YBCO and AgYBCO materials. U_eff is higher in silver doped samples in the high temperature region T≥60K.     

Fabrication of Bi2Sr2CaCu2O x and Bi2Sr2Ca2Cu3O x Superconductor Thick Films by Using Cu-Free Precursors on Ni Substrates

We studied the formation of Bi₂Sr₂CaCu₂O _x (Bi-2212) and Bi₂Sr₂Ca₂Cu₃O _x (Bi-2223) thick films in a heat treatment process of the Ni-sheathed Bi-Sr-Ca-Cu-O (BSCCO) system. Cu was electrodeposited initially on the Ni substrates (Cu/Ni). Well-oriented Bi-2212 superconductor thick films were formed successfully on Ni tapes by liquid reaction between Cu-free precursors and Cu/Ni tapes. However, only a small amount of Bi-2223 was formed. Thick films were prepared by screen-printing with Bi₂O₃, SrCO₃, and CaCO₃ powders on Cu/Ni tapes and heat treating them. Heat treatment was performed in the temperature range of 750–850°C in a tube furnace for several minutes to hours. The phases and the microstructures of the high temperature superconductor thick films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electrical properties were examined by the standard four-probe method. At the heat treatment temperature, the specimens were in a partially molten state during reaction between the oxidized copper layer and the screen-printed precursors on the Cu/Ni tapes.     

Hot electrons and phonons under high intensity photoexcitation of semiconductors

It has become well established during the last few years that intense photoexcitation of a semiconductor leads to the heating of the carriers and the generation of nonequilibrium phonons. These phenomena which result from the relaxation of photoexcited carriers to the band extrema by interaction with other carriers and by emission of phonons, are reviewed in this paper. At relatively low intensities (<10⁵W/cm² for GaAs) the photoexcited carrier distribution is Maxwellian with a carrier temperature T_e different from the lattice temperature. T_e as high as 150K and effective phonon temperatures as high as 3700K have been observed in GaAs. The observed variation of T_e with excitation intensity leads to the conclusion that in semiconductors like GaAs the polar optical mode scattering is the dominant energy loss mechanism from the electron gas to the lattice. Similar results are obtained in CdSe and CdS. At higher intensities (>10⁵W/cm² for GaAs), the carrier dist0ribution becomes non-Maxwellian for reasons not well understood at present. We will also discuss some recent measurements of variation of T_e with excitation wavelength and of the transmission spectra of photoexcited GaAs.     

High-temperature superconducting magnet motor demonstration

A superconducting field winding, assembled with coils constructed with Bismuth 2223 HTS wire, has been successfully operated in an electric homopolar motor. At a field winding temperature of 4.2 K, the motor produced 125 kW of output power. At a temperature of 28 K, the motor power developed was 91 kW. These measured power levels are the highest that have been produced by an electric motor containing a HTS field winding