ac losses in Nb3Ge composite tape conductors

ac losses of Nb₃Ge tapes prepared by chemical vapour deposition have been measured between 4 and 16 K. Several tapes were observed to exhibit complete shielding of the normal substrate by the Nb₃Ge coating, which is continuous around the tape circumference. These tapes had measured losses that were acceptably low for power transmission up to 14 K. Another class of tapes showed high losses characteristic of ohmic currents flowing through the normal substrates. Microscopic examination revealed continuous longitudinal edge cracks for these samples. A correlation observed between ‘edge cracking’ and Nb³Ge coat thickness is discussed in terms of the differential contraction from coating temperature between the Nb₃Ge and the substrate in a completely enclosed geometry.     

Fabrication and Electromechanical Characterization of Silver-Clad BSCCO Tapes

The powder-in-tube technique consisting of industrial processes such as wire drawing and rolling has been widely used to fabricate superconducting tapes. In the present investigation a novel technique was adopted to fabricate BSCCO 2223 tapes. Instead of wire drawing, the silver billet was reduced in size by groove rolling. Stress conditions during groove rolling were analyzed and appropriate changes were incorporated to optimize the deformation process. Subsequent thermomechanical treatment resulted in tapes with average critical current densities of 18,000 A/cm². Phase development and microstructural evolution during the thermomechanical treatment were studied using XRD, SEM, and TEM. The electromechanical properties of monofilament and composite BSCCO tapes were evaluated by subjecting them to in situ tensile tests. The strain tolerance of the composite was found to be better than that of the monofilament BSCCO tape.     

An evaluation of metal and semiconductor films formed by ionized-cluster beam deposition

The mechanism of film formation and the properties of films deposited by the ionized-cluster beam technique were investigated. In this technique, strong adhesion of the film to the substrate and good crystalline deposition are expected.A high adhesive strength of over 100 kg cm^-2 for Cu films on glass substrates was obtained in the experiments. The migration of adatoms consisting of ionized and neutral clusters was observed on the substrte surface. This effect, called the migration effect, can be considered to characterize film formation by cluster beam deposition and to produce good crystalline films. Si single crystals were obtained on Si substrates. A p-n junction photodiode was fabricated by the deposition of n-type Si on a p-type substrate. The diode shows improved spectral sensitivity in the UV region compared with that of commercially available solar cells.     

Test Coils Made of Tl-1223 Tapes

Silver sheathed Tl-1223 tapes were prepared by a powder-in-tube process. The critical current density of short samples was 18 kA/cm² at 77 K. Longer tapes up to 1.2 m, prepared by sequential pressing, had a critical current density of 12 kA/cm². From these tapes we have wound two coils. A solenoid coil with 5 windings was made of 8 tapes with a total length of 4.5 m. At 77 K the critical current of the coil was 23 A in the self generated magnetic field (18 Gauss at the centre of the coil). Using an iron yoke the critical current remained at 22 A while the generated magnetic field increased to 120 Gauss. A pancake coil with 15 windings, made of 5 tapes with a total length of 5 m, generated a magnetic field of 149 Gauss at the critical current of 12 A. From measurements of the critical current density of our tapes in applied magnetic fields, we conclude that coils made of Tl-1223 tapes can be used to generate higher magnetic fields at 77 K.     

Methods to Produce Tl(1223) Tapes with Improved Properties

Different methods to prepare superconducting mono- and multifilamentary T1(I223) tapes were developed. High-purity ceramics could be produced by ex-situ or in-situ reaction under a high isostatic gas pressure. Tapes prepared by the PIT method and an in-situ reaction carried out close to the melting point of the oxide were relatively dense and textured. Interesting results were obtained for samples where part of the oxygen atoms in Tl(1223) had been substituted by fluorine. Critical current densities up to 20'000 A/cm² (77 K, 0 T) were reached for tapes produced by the PIT method. Well textured samples were obtained by electrophoretic deposition, the deposited Tl(1223) grains being produced by a synthesis involving substantial melting.     

Effect of Initial Surface Quality on Final Roughness and Texture of Annealed Ni-5at.%W Tapes Coated with a Gd2Zr2O7 Buffer Layer

Surface roughness of Ni-5at.%W tapes in cold-rolled and annealed conditions after subsequent deposition of a Gd₂Zr₂O₇ buffer layer has been studied as a function of the polishing grade, taking grain boundary grooving into account. It is found that annealing decreases the initial mean surface roughness achieved by mechanical polishing of the cold-rolled material, except after very fine polishing. Furthermore, compared to the surface of the tape annealed after fine polishing, the mean roughness slightly increases after the deposition of the buffer layer. Grain boundary grooving was found to impose a lower limit for the mean surface roughness. In the annealed tapes, the fraction of orientations within 5^○ from the ideal cube orientation was observed to be very sensitive to the surface roughness before annealing.     

Preparation of Nb<Subscript>3</Subscript>Al Superconducting Tapes by a Powder-in-Tube Method Combined with Hot-Pressed Sintering

The precursors of Nb-Al tape were fabricated by a powder-in-tube (PIT) technique. Supersaturated solid solutions of Nb-Al powder were obtained by high-energy ball milling, followed by a powder-in-tube process to prepare series of Nb₃Al precursor tapes. Compared with sintering under normal pressure, the hot-pressing sintering greatly increased the critical current density of the tapes. The results showed that the Nb₃Al tapes prepared via the powder-in-tube method and hot-pressed sintering could significantly improve the J _c performance, and the value of J _c (8 K, 0 T) was higher than 6 × 10⁵ A/cm². The Nb₃Al tapes after pressing under the pressure of 20 MPa and sintering at 950 ^°C for 3 h had the best magnetic flux pinning performance, and the sample reached the maximum flux pinning force (F _p,max) of 3.28109 N/m³ in the magnetic field of 2.2 T.     

Chemical vapour deposition of superconductors

Chemical vapour deposition processes (CVD) can produce metastable fine-grained materials as well as epitaxial coatings and can have a very large throwing power depending on the process parameters. Therefore, CVD is an prospective method to deposit high-temperature superconducting materials withT _c⩾10 K. One of the first superconductors which were produced was Nb₃Sn on tapes and single wires. This superconducting material is, however, today produced by metallurgical methods. Since the detection of Nb₃Ge, CVD has become for these coatings the main method of production for the following reasons: high deposition rates, possibility to dope the material by addition of further doping gases to the CVD-process, continuous process. These coatings were deposited on tapes. For the first time the large throwing power of the CVD process was utilized for the deposition of B1 -NbC _x N _y , on carbon fibre bundles. This opens the possibility to produce multifilamentary structures used for magnetic applications. The structure of the coating can be varied by changing the gas properties, by addition of further gases, by an ultrasonic field, by ignition of a gas discharge and by multi-layering. CVD could also be a prospective method for producing the new class of superconductors withT _c⩾30 K.     

Hg-1223 Thick Film on Flexible Ni Substrates

Hg-1223 thick films on flexible Ni substrate coated with Cr(Ag,Pd) have been fabricated by a two-step spray/press process. The effect of possible interfacial diffusion between Hg-1223/Ni has also been examined. By properly adjusting the processing parameters, a self-field J_c 7×10⁴ A/cm² at 77 K has been achieved inthese Hg-1223 thick-film tapes, comparable to that of the well-known BSCCO tapes prepared by the more complex powder-in-tube technique.     

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.     

Development of Ag-Sheathed Bi(2223) Tapes with Improved Microstructure and Homogeneity

The development of the fabrication process of Ag-sheathed Bi(2223) tapes has been carried out in order to improve their transport and mechanical properties, as required by the power applications which are so far under study. Critical current density values of 28 kA/cm² at 77 K have been achieved on long multifilamentary Bi(2223) tapes, with a fabrication process that has been successfully employed in the fabrication of samples longer than 50 m. The microstructure and homogeneity of Ag-sheathed multifilamentary Bi(2223) tapes has been markedly improved by employing an alternative deformation technique. In a substantial part of the fabrication process, swaging, drawing, and rolling have been replaced by deformation with an active turks-head machine, which allows the deformation of rectangular shaped wires. At present, critical current densities in excess of 25 kA/cm² at 77 K have been achieved on long samples prepared with this technique. Moreover, innovative filament configurations have been employed for the fabrication of square-shaped Bi(2223) wires with reduced anisotropy and with critical current densities exceeding 20 kA/cm² at 77 K.     

All-poly(ethylene terephthalate) composites by film stacking of oriented tapes

Self-reinforced polymer or all-polymer composites have been developed to replace traditional fibre reinforced plastic (FRP) with good interfacial adhesion and enhanced recyclability. Poly(ethylene terephthalate) (PET) is one of the most attractive polymers to be used in these fully recyclable all-polymer composites, in terms of cost and properties. In this work, all-PET composites were prepared by film stacking of oriented PET tapes. A processing temperature window was determined by a series of tests on PET tapes and co-PET films, including DSC and T-peel tests. Tensile properties of PET tape, co-PET film and all-PET composites are reported and compared with a commercial co-extruded PURE^® polypropylene tape. The effect of compaction temperatures and pressures on tensile properties of all-PET composites was investigated to explore the optimum processing parameters for balancing good interfacial adhesion between tapes and residual tensile properties of PET tapes.     

Fabrication and Properties of MgB2 Coated Superconducting Tapes

We present the formation of MgB₂ coatings by simple and novel aerosol deposition technique which has a potential to escalate towards the fabrication of long superconducting tapes. The thin MgB₂ coatings were produced by using pre-synthesized MgB₂ powder. The ability of this technique to form a precursor powder in a thin film form has greatly reduced the intricacies involved in the synthesis of MgB₂ by other techniques like hybrid physical chemical vapor deposition etc. The as-synthesized thin films were characterized by the x-ray diffraction technique to study the structural properties. The thin films were found to be x-ray amorphous in nature depicting the formation of frustrated structure which showed a superconducting transition onset at around 36 K.     

Deposition and characterization of highly p-type antimony doped ZnTe thin films

We report deposition of highly p-type antimony doped ZnTe films onto glass substrates using ZnTe and Sb effusion cells in ultra-high-vacuum environment of an MBE system. It was found that the widely-used conventional co-evaporation technique does not produce highly p-type films. Through a series of deposition experiments, a 4-step method consisting deposition of the layers followed by a post annealing process was developed. The maximum carrier concentration was 3 × 10¹⁹ cm⁻³ which is the highest reported for ZnTe:Sb films. The surface morphology and the structure of the films were analyzed using AFM and XRD. Electrical properties of Sb doped films were investigated by four-point probe and room temperature Hall effect measurements.     

Preparation of NiO Buffer Layer by Direct Oxygenation of Ni Tape

Recently, Nickel-based alloys are widely used for textured substrates tapes. However, the deposition of oxide buffer layers on nickel tapes is required to stop the diffusion of nickel from the tape to superconducting layer and to improve the mismatch between the substrate and superconducting film. Biaxially textured NiO buffer layer is easily formed on cubic textured nickel tape by the technique called surface oxidation epitaxy. In this work, we developed a direct oxygenating method to make the NiO buffer layer on nickel tape. The nickel tape was directly oxygenated in different atmospheres. It is found that in inert atmosphere, a high quality NiO layer can be formed on the surface of the tape. The oxygenating conditions, including atmospheres and temperatures, and their influence on the structure of the NiO buffer layer, were studied in detail and discussed.     

Preparation and characterization of silver clad (Bi,Pb)-Sr-Ca-Cu-O 2223 superconducting tapes with high critical current density

Silver clad Bi-2223 tapes with consistently high critical current densities of over 30,000 A/cm² at 77 K and zero field were prepared by powder-in-tube (PIT) technique. Powder XRD, electron microscopy, a.c. susceptibility and critical current measurements were used to study the phase assemblage, microstructure and transport properties of these tapes at various stages of processing. The precursor powder for PIT process was prepared by a sol-gel route by acrylate method using freshly prepared nitrates of Bi, Pb, Sr, Ca and Cu. The carbon content in the powder was minimized by subjecting it under dynamic vacuum calcination followed by heating in free flow of oxygen for long durations with intermittent grindings. The choice of initial stoichiometry, high reactivity of the precursor, effective removal of carbon, choice of phase assemblage at the filling stage and the multistage thermomechanical processing at optimized conditions were found to be responsible for the high critical current density. The work was done under the National Superconductivity Programme funded by the Department of Science and Technology (DST), New Delhi.     

Electro-spark deposition of Fe-based amorphous alloy coatings

A simple and effective surface coating technique, electro-spark deposition (ESD), has been used to produce amorphous alloy coatings. Fe-Cr-Mo-Gd-C-B amorphous alloy rods produced by copper mould casting were used as electrode to produce coatings onto 304 stainless steel substrate. Classical X-ray diffraction (XRD), glancing angle XRD (GAXRD), and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis indicate that the coatings have an average thickness of ∼ 30 μm, show an amorphous structure, and are metallurgically bonded to the substrate. Microhardness tests showed that the coating layer has a high hardness of 1542 kg/mm², implying a much improved wear resistance on surface of stainless steels.     

Variance of Critical Current Density with Microstructure by a Special Process

A promising method —“powder in tube”technique was used to fabricate Ag-sheathed Bi-2223superconductive tapes with high critical current density.After a combination processing of pressingand subsequent heat treatment,we obtained tapes with high degree of texture,good compactionand uniform properties.At 77 K in zero field,J_C was higher than 1×10~4 A/cm~2 while the highestJ_C was 1.69×10~4 A/cm~2.SEM and XRD was used to detect the tapes texture,and the relationship be-tween J_C and the degree of texture is discussed.In addition,the reason for obstacling the improvementof J_C is also investigated.     

The influence of the tape-casting process parameters on the geometric characteristics of SiC tapes

The production process of the tape-casting technique applied to the production of porous ceramic membranes, suitable for the separation of different chemical species, has been explored, with emphasis on the influence of the operative parameters on the macrogeometric characteristics of the manufactured ceramic porous tapes. The effects of the slurry casting rate, the doctor blade height and the mean particle diameter have been studied and the results reported. The characterization analyses indicate the degree of reliability of this technique to produce planar SiC components and the importance of the particle size in enhancing the structural homogeneity. A study on the variance analysis (ANOVA) determined that important operative parameters influencing the thickness of the planar components are, in order of importance, the doctor blade height and the slurry casting rate. The squared terms H_r^{poststaggered2}, V_p^{poststaggered2} and the interaction term V_pH_r have very little influence on the value of log(S).     

Temperature oscillations in thin metallic tapes with transport current at liquid nitrogen temperature

Temperature oscillations were observed in thin metallic tapes cooled by liquid nitrogen at current densities above 10⁸ A/m². A boiling hysteresis in the transition region between conductive heat transfer and nucleate boiling caused a temperature decrease and a change in the resistance of the tape. The differential resistance was negative and the current voltage curve had a non-linear character. The amplitude of the resistance change and character of the oscillations depended on the type of metal used, current ramp rate, geometry of the tape, and on its position with respect to the liquid surface. The effect was most pronounced in Ni tapes; the resistance change during the temperature drop achieved a value of about 30%. Dumped and un-dumped oscillations were observed after the temperature drop.