Reactive sintering of Bi2Sr2CaCu2Ox superconductors

Three phase assemblages were used to produce Bi₂Sr₂CaCu₂O_x (2212) during sintering: a mixture of 20% Ca-rich 2212+80%2212, partially synthesized 2212, and Bi₂Sr₂CuO_x (2201)+(1/2 Ca₂ CuO₃+1/2 CuO) (denoted 0011). The mixture of 2201+0011 produced highly pure 2212 within 50 h of heating in air at ≈850°C. Ag tubes were filled with a mixture of 2201+0011 and worked into tapes by a powder-in-tube process. Heat treatments produced microstructures consisting of small, highly textured 2212 grains. T_c values were ≈70K. Transport J_c values at 4.2K were ≈10⁴ A/cm².     

Superconducting Properties of (M x /YBa2Cu3O7−δy )N Multilayer Films with Variable Layer Thickness x

The superconducting properties of (M _x /YBa₂Cu₃O_7−δy )_N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y₂BaCuO₅ (211), Y₂O₃, BaZrO₃, CeO₂, SmBa₂Cu₃O_7−δ (Sm123), brown-phase La₂BaCuO₅ (La211), and MgO. Multilayer (M _x /YBa₂ Cu₃O_7−δy )_N structures were grown by pulsed laser deposition onto SrTiO₃ or LaAlO₃ single-crystal substrates by alternate ablation of separate YBa₂Cu₃O_7−δ (123) and M targets, at temperatures of 750°C to 790°C. The x layer thickness was varied from 0.1 nm to 4.5 nm, and the y 123 layer thickness was kept constant within a given range of 10 to 25 nm. Different M phase and x layer thicknesses caused large variations of the microstructural and superconducting properties, including superconducting transition (T _c), critical current density as a function of applied magnetic field J _c(H), self-field J _c(77 K), and nanoparticle layer coverage. Strong flux-pinning enhancement up to 1 to 3x was observed to occur for M additions of 211 and BaZrO₃ at 65 to 77 K, Y₂O₃ at 65 K, and CeO₂ for H < 0.5 T. BaZrO₃ had a noticeably different epitaxy forming smaller size nanoparticles ∼8 nm with 3 to 4x higher areal surface particle densities than other M phases, reaching 5 × 10¹¹ nanoparticles cm⁻². To optimize flux pinning and J _c (65 to 77 K, H = 2 to 3 T), the M layer thickness had to be reduced below a critical value that correlated with a nanoparticle surface coverage <15% by area. Unusual effects were observed for poor pinning materials including Sm123 and La211, where properties such as self-field J _c unexpectedly increased with increasing x layer thickness.     

Microstructure and properties of Tl0.75Bi0.25Sr1.6Ba0.4Ca2Cu3Ox tapes

Ag-clad Tl_0.75Bi_0.25Sr_1.6Ba_0.4Ca₂Cu₃O_x tapes were fabricated by a powder-in-tube technique. The starting powders consisted of Bi₂Sr₂CaCu₂O_x plus simple oxides. The tapes were heated in flowing O₂ at 835–865 °C for 7.5–10 h. Room-temperature pressing at 1.0–1.5 GPa produced tapes with denser, more phase-pure cores, and transport critical current density J_c at 77 K in self-field that was increased by a factor of ≈2. The maximum J_c value of 6 × 10³ A/cm² was obtained with heating at 840 − 850 °C for ≈ 10 h, with three intermediate pressing steps. The cores of the best tapes were still rather porous and contained significant concentrations of nonsuperconducting phases.     

Effects of shock-induced defect density on flux pinning in melt-textured YBa2Cu3O7−δ

To introduce a high density of homogeneously distributed defects in YBa₂Cu₃O_7−δ (Y123), melt textured samples were shock-compacted at pressures up to 12.6 GPa (126 Kbar) at orientations favorable to slip along the basal planes.¹ Shock compressing melt-textured Y123 results in a nearly uniform detect density which is two to three orders of magnitude higher than in unshocked melt-textured material. However, the intergranular critical current density in bulk samples (J_c ^b) decreases by two orders of magnitude in the as-shocked state. This decrease in J_c ^b is attributed to microfractures. However, if the shocked disk is annealed in O₂ then ground, sieved, and magnetically aligned, J_c for H τ c-axis (J_c ^ab) is enhanced two to three times over the unshocked value. This indicates that the increase in dislocations density does increase flux-pinning.     

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.     

Development of needle-shaped grains from Bi2Sr2CaCu2O8+x with high critical current density suitable for making superconducting wires

We present a grain microstructure for Bi(2212) consisting of only giant needle-shaped grains of around 1.5 mm length and 100 μm diameter. We study the structural and chemical changes suffered by a conventional ceramic Bi(2212) sample in the course of the thermal treatment used to obtain those giant needle-shaped grains. For that, different samples of the same batch were treated with incomplete thermal treatments, and the resulting samples were analysed by using scanning electron microscopy (SEM), optical microscopy, energy dispersed spectroscopy (EDS), inductively coupled plasma (ICP) and X-ray diffraction (XRD). To verify the superconducting nature of the needle-shaped grains, we have performed magnetization, resistivity, and critical current measurements on the original ceramic sample, and on that formed as giant needle-like grains. The critical temperature of these last grains is nearly the same as that of the ceramic sample (T_c∼90 K), which is a high value for the Bi(2212) compound. The critical current density (J_c) of the needle-shaped grains is around 2500 A/cm² at 77 K and in absence of applied magnetic field, a value comparable with that presented for the best wires and thick films. Not only are the shape and the size of these grains very suitable for making superconducting wires, but also the superconducting properties, T_c and J_c, are both high enough to be confident about the possibility of improving the actual Bi(2212) superconducting wires for high current applications.     

Epitaxial Growth of High-J c NdBa2Cu3O7−δ Films on RABiTS by Pulsed Laser Deposition

NdBa₂Cu₃O_7−δ (NdBCO) films were grown on rolling-assisted biaxially textured substrates (RABiTS) via pulsed laser deposition. c-Axis-oriented epitaxial NdBCO films with high performance were obtained under optimal deposition conditions. Transmission electron microscopy analysis shows that the NdBCO film grown on RABiTS has a clear interface with a CeO₂ cap layer and a nearly perfect lattice structure. The NdBCO film exhibits higher T _c of 93.7 K and better in-field J _c in magnetic fields and at all field orientations, compared to pure YBCO films.     

Aspects of the current understanding of the supercurrent transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed Tapes—the “railway-switch” model

The “railway-switch” model describes the superconducting current transport in (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ silver-sheathed tapes under the basic assumption that small-angle c axis tilt grain boundaries (“railway-switch”) constitute strong intergrain links for the supercurrent in the textured filament [B. Hensel, J.-C. Grivel, A. Jeremie, A. Perin, A. Pollini, and R. Flükiger,Physica C 205, 329 (1993)]. We give an overview of the model and some recent experimental results with the objective to identify the mechanisms that limit the critical current density. The measurements have been performed on monofilamentary “powderin-tube” samples [J_c(T < 77K, B < 0 T) < 20..30 kA/cm²] that were prepared in long lengths by rolling as the only tape-forming process. We conclude that the low intragrain critical current density j_c ^c along the c axis (or the even lower critical current density j_c ^t across twist boundaries or intergrowths) is the dominant limitation for the transport critical current in high-quality tapes. We discuss possible starting points for a performance improvement of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ silver-sheathed tapes for applications. On leave from Consorzio INFM, Universitá di Genova, Italy.     

Microstructure of high Jc NdBa2Cu3Ox/Nd4Ba2Cu2O10 composite superconductor

Single crystalline NdBa₂Cu₃O_x (Nd123) superconductors with dispersed Nd₄Ba₂Cu₂O₁₀ (Nd422) particles were produced by the floating zone partial melting and solidification (FZPMS) method. The initial composition of the precursor material was Nd_1.8Ba_2.4Cu_3.4O_x with or without 0.1 wt% Pt addition. FZPMS was carried out in a low oxygen partial pressure atmosphere. Microstructure of quenched samples were investigated by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Furthermore, superconductive properties were measured by superconducting quantum interference device (SQUID). The results of SQUID measurements indicate that the critical temperature (T_c) of the samples with Pt addition so produced with the oxygen heat treatment (623 K for 300 h) was 94 K and the critical current density (J_c) was 2.3×10⁴ A/cm² at 77 K, 0.2 T.     

Superconducting and Transport Properties of (Bi-Pb)-Sr-Ca-Cu-O with Nano-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Additions

The effect of nano Cr₂O₃ additions in (Bi, Pb)-Sr-Ca-Cu-O superconductors using the coprecipitation method is reported. Nano Cr₂O₃ with 0.1, 0.3, 0.5, 0.7, and 1.0 wt.% were added to the (Bi, Pb)-Sr-Ca-Cu-O system. The critical temperature (T _c) and transport critical current density (J _c) were determined by the four-point probe technique. The phases in the samples were determined using the powder X-ray diffraction method. The microstructure was observed by a scanning electron microscope and the distribution of nano Cr₂O₃ was determined by energy-dispersive X-ray analysis (EDX). The maximum T _c and J _c were observed for the sample with 0.1 wt.% nano Cr₂O₃. The variation in the J _c of all the samples was explained by the effective flux pinning by nano Cr₂O₃ in the samples. Using the self-field approximation together with the dependence of J _c on temperature, the characteristic length (L _c) associated with the pinning force was estimated to be approximately the same as the average grain size in all the samples.     

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.     

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.     

The Effect of Doping with the Polymer Metallic Complex Poly Zinc Acrylate on Superconducting Bulk MgB<Subscript>2</Subscript>

In this work, the effect of doping with 5% to 20% poly zinc acrylate (PZA) complexes on MgB₂ phase formation was studied. The changes of the lattice parameter and the amount of substituted carbon in the microstructure as well as the critical temperature and the critical current density (J _c) of bulk MgB₂ have been studied systematically. Both differential thermal analysis–thermogravimetric (DTA–TG) analysis and infrared (IR) spectrometry results show that the PZA was decomposed between 400°C and 600°C, and the main decomposition products were carbonized polymers and ZnO. The lattice parameter a in all PZA-doped MgB₂ samples was decreased by different degrees, which indicates gradual substitution of carbon on the boron sites in the MgB₂ crystal structure. Compared with the undoped sample, the J _c values of all the doped samples were enhanced by about an order of magnitude under high magnetic fields. The J _c value of the MgB₂ sample with 20% PZA doping sintered at 800°C for 1 h reached 8 kA/cm² at 5 K and 8 T. Transmission electron microscopy (TEM) images revealed a considerable amount of second-phase particles of ~5 nm size, and a high density of dislocations was observed in the MgB₂ grains.     

Effect of Heat Treatments on the Structural and Magnetic Properties of La-Co Substituted Strontium Ferrite Films

A sputter-deposited strontium ferrite film with perpendicular anisotropy has been developed. The film, composed of La_0.33Sr_0.67Co_0.25Fe_11.75O₁₉, has been fabricated directly on quartz glass substrates by radio frequency magnetron sputtering with various heat treatments. The structural and magnetic property dependence of those films on heat treatments has also been studied. The optimized condition is the heat treatment of in situ heating at 400°C and post-annealing at 850°C–900°C. When post-annealing temperature exceeds 900°C, parasitic phases of γ-Fe₂O₃ and LaFeO₃ appear and gradually increase; meanwhile, the magneto plumbite phase gradually decreases. High c-axis perpendicularly oriented films with the coercivity (4148 Oe), remanence squareness ratio (0.89) and perpendicular magnetic anisotropy energy density (1.65 × 10⁶ erg/cm³) are achieved, which is attributed to the single magneto plumbite phase with compact platelet grains and almost complete (0 0 l) texture of the c-axis normal to the film plane.     

The interactions between Si/Co films and GaAs(001) substrates

Interfacial reactions of Si/Co films on (001) oriented GaAs substrate, in the temperature range 300–700°C for 30 min, have been investigated using a combination of x-ray diffraction, Auger electron spectroscopy, and transmission electron microscopy. Cobalt starts to react with GaAs and Si at 380°C by formation of Co₂GaAs, and Co₂Si phases, respectively. At 420°C, the entire layer of Co is consumed, and the layer structure is observed with the sequence Si/CoSi/CoGa(CoAs)/Co₂GaAs/GaAs. Contacts produced in this annealing regime are rectifying and the Schottky barrier heights increase from 0.69 eV(as-deposited state) up to 0.81 eV (420°C). In the subsequent reaction, CoSi grows at the expense of the decompositions of CoGa and CoAs at 460°C. In addition, the ternary phase also is decomposed and only the CoSi phase remains upon the GaAs surface at 600°C. Contacts produced at higher temperature regime (>460°C) have low barriers. The interface between CoSi and GaAs is stable up to 700°C. The results of interfacial reactions can be understood from the calculated Si−Co−Ga−As quaternary phase diagram.     

Techniques for texturing ceramic superconductors from an amorphous precursor

Amorphous Bi₂Sr₂CaCu₂O₈ (Bi-2212) was crystallized under a uniaxial load of 1500 N at temperatures up to 880°C without Ag and 850°C with Ag to induce texture. Well textured samples (19 mm in diameter and 0.15 mm thick) were obtained for samples heated to within 99% of the melting temperature and quenched for samples with and without Ag. The rate limiting step for formation of the Bi-2212 from the amorphous precursor is a diffusion limited intercalation of (SrCa)CuO₂ into Bi₂Sr₂CuO₆ (Bi-2201) at T>600°C. Samples rapidly heated to within a few degrees of the melting temperature and quenched show 20 to 30% Bi-2201 layers within the Bi-2212 structure. Annealing these samples at 850°C for >60 h eliminates most of the Bi-2201 layers resulting in sharper superconducting transitions and higher intracrystalline critical currents with no adverse effects on grain size or texture. It is more difficult to remove the intercalations in the Ag added material.     

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.     

Optimization of Phase Formation and Superconducting Properties in MgB<Subscript>2</Subscript> Prepared by Phase Transformation from MgB<Subscript>4</Subscript>

In this work, polycrystalline MgB₄ was prepared, and then the presence of MgO in the powder was reduced by acid leaching. To synthesize MgB₂, the MgB₄ powder was reacted with Mg at temperatures from 650°C to 950°C for 4 h and 8 h. Using the Rietveld method, the MgB₂ phase was estimated to be in greater proportion over the sintering temperature range of 650°C to 750°C, above which it decreased rapidly, accompanied by the formation of more of the MgB₄ phase. Scanning electron microscopy showed that the samples have reduced porosity compared with those synthesized by the direct reaction of (Mg + 2B). Upon increasing the sintering temperature, the superconducting transition temperature decreased, which is attributed to lattice distortion. The results of this work demonstrate that control of heat treatment is essential to optimize the weight fraction of the MgB₂ phase and thereby increase the critical current density, J _c. The value of the magnetic J _c (5 K, 1 T) for the sample sintered at 750°C for 4 h is estimated to be 1.00 × 10⁵ A/cm².     

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.     

Growth of high quality YbBa2Cu3O7−δ thin films by pulsed laser deposition

YbBa₂Cu₃O_7−δ (Yb-123) films are deposited for the first time using Pulsed Laser Deposition (PLD) method at three different substrate temperatures, viz. 675°C, 700°C and 725°C. Films are characterized using XRD, dc electrical resistivity, critical current density (J_c) and microstructural study by Atomic Force Microscopy (AFM) techniques. It is found that 700°C is the optimum growth temperature for growing high quality Yb-123 films. The best T_c and J_c values obtained at optimum growth conditions are 88 K and 2.6×10⁶ A cm⁻² at 77 K, respectively. AFM photographs provide evidence in confirming the relation between growth temperature and superconducting properties.