G. K. Padam

Studies on solution‐grown thin films of ZnxCd1−xS

Structural, optical, and electrical properties of thin films of ZnxCd1−xS (0≤x≤1.0) prepared by the solution‐growth technique are reported as a function of x. As‐grown films are composed of a hexagonal phase in the range 0≤x≤0.8 and of cubic phase for x=1.0. The lattice parameters a and c vary with x following Vegard’s law. The band gaps for as‐grown ZnxCd1−xS films are found to vary linearly with x from 2.40 eV for CdS to 3.6 eV for ZnS. The resistivity increases from 109 to 1012 Ω cm in the whole range 0≤x≤1.0. The effect of indium doping and H2 annealing on the above properties is also investigated.

Study of intrinsic defects in vacuum/air annealed CuInSe2

Abstract This paper describes the influence of vacuum/air annealing and the Cu: In ratio on defects produced in chemically prepared CuInSe 2 thin films/powders. Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques were used for analysis. Two isotropic EPR signals A (g = 2.0030) and B (g = 2.1642) obtained in both air and vacuum annealed samples have been ascribed to In Cu donors and V Cu acceptors, respectively. On annealing CuInSe 2 in air, the concentration of In Cu donors decreases, V Cu acceptor concentration increases, increase in p-type nature and resistivity decreases. However, on increasing the Cu: In ratio, the concentration of both types of defects decrases but p-type nature increases with decreasing resistivity. Air annealing of the samples also induces the formation of extraneous phases like Cu 2 Se, SeO 2 and In 2 O 3 . The near-stoichiometric and Cu-rich samples have been found to be chalcopyrite, whereas Cu-poor samples have the tendency to be sphalerite. All the results obtained can be understood in terms of In Cu donor-oxygen interaction and the formation of V Cu acceptors.

Study of silver addition to Y1Ba2Cu3O7−y screen printed thick films

A study of the effect on the superconducting and microstructural properties of YBa2Cu3O7−x screen printed thick films as a function of silver content (in the form of Ag and Ag2O) from 0 to 40 wt. % by employing different experimental techniques has been carried out. It has been found that silver improves the sinterability and intergrain connectivity and reduces the normal state resistivity. The superconducting transition temperature of the above films was found to be in the range of 82 to 91 K. Our results show that a small addition of silver (Ag or Ag2O) helps in enhancing the superconducting properties. However, large amounts lead to formation of 211 phase as the decomposed product and thereby deteriorate the quality of the superconductor. Silver has been found to remain as metal in the samples for the entire range. Further, in light of our results, it seems that an addition of Ag metal is more beneficial than the addition of Ag2O.

Non-resonant microwave absorption studies in Bi1.6Pb0.4Sr2Ca2Cu3O10+x

Abstract Non-resonant microwave absorption (NRMA) studies have been carried out on Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+ x pellets sintered for different durations varying from 24 to 168 h by using EPR technique. A detailed study of the temperature dependence of the NRMA signal amplitude, peak-to-peak width, hysteresis area has been carried out. Apart from showing a flux line lattice (FLL) melting temperature T m ( B ) (60–70 K), it is interesting to note that hysteresis loops showed the anomalous magnetic behaviour in the entire temperature regions even far below the fluctuation regime. The results are discussed in terms of several current models for the NRMA effects as well as for its anomalous magnetic hysteresis behaviour.

A comparative study of rapid and slow furnace cooling effects on the superconducting properties of Pb-free and Pb-doped Bi-2212 HTSC

Abstract A study of the Pb-doped Bi-2212 phase in comparison to that of the Pb-free Bi-2212 phase under two different conditions: (i) rapid cooling (RC) and furnace cooling (FC), and (ii) variation of sintering time from 12 to 144 h has been carried out. The Pb-doped samples showed a change, while the Pb-free samples did not show any change in the superconducting behaviour with increasing sintering time under identical conditions of RC and FC. For all the sintering durations, the Pb-free samples always showed T c (FC) > T c (FC), while the Pb-doped samples showed the opposite trend, that is, T c (RC) T c (FC) for shorter sinterings (12 to 36 h), whereas, an identical trend, that is, T c (RC) > T c (FC) for longer sinterings (48 to 144 h). Detailed XRD, TEM, SEM and EDAX studies suggest that this changing behaviour of the Pb-doped Bi-2212 samples may probably be due to changes occurring in the band structure and in the microstructure affecting electronic properties of the material depending upon the site occupied by Pb under RC and FC regimes, with increasing sintering time.

Transformation of 2212 to 2223 phase in the Bi-Sr-Ca-Cu-O system

Single phase Bi2Sr2Ca1Cu2O8 (2 2 1 2) superconductor with T°c around 85 K was transformed to nearly a single phase Bi2Sr2Ca2Cu3O10 (2223) having T°c around 115.5 K by reacting at 860°C with CaO and CuO (1 mol each of CaO and CuO, per formula composition Bi2Sr2Ca1Cu2O8) with addition of Pb for stabilization. A sample prepared under identical conditions but using the Bi2Sr2Ca2Cu3Oy nominal composition, however, showed only a very small amount of 2223 phase with T°c around 107 K. The corresponding changes in the superconducting phases and morphologies were also systematically investigated and found that uniform and sufficient supply of Ca and Cu ions in the correct proportions at early stages in the processing might have led to a key role during the subsequent 2212–2223 phase transformation.

Synthesis and characterization of Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O superconductors from the oxine co-precipitation method

Oxide superconductors in the Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O systems with Tc degrees of 93.6 K and 115 K, respectively, have been prepared by the co-precipitation process via the oxine route using nitrates as the starting materials. Chemical homogeneity, stoichiometry and fine particles of the precursor obtained by this process permit a relatively short time for the production of Y- and Bi-based superconducting compounds compared with that obtained by the conventional solid state reaction method. The resulting densities of the sintered samples were typically greater than 90%. X-ray diffraction analysis, energy dispersive spectral analysis and scanning electron microstructural analysis are also reported in this study.

Preparation and characterization of Ag-added Bi1.84Pb0.4Sr2Ca2.2Cu3O10+x bulk tube conductors for cryogen free superconducting magnet

Bulk tube conductors of Bi1.84Pb0.4Sr2Ca2.2Cu3O10+xwith addition of silver varying from 0 to 25 wt% (not reported earlier) were systematically studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical transport and a.c. susceptibility techniques. The tube conductors formed by cold isostatic pressing (CIP) of the powders obtained from spray drying method have been made successfully. It was found that Ag addition has not only affected the formation of the desired Bi-2223 phase and the microstructure of these large bulk tube samples thereby influencing on the critical current (Ic),it also reduces the contact resistance to minimize the cryogen losses. These variations have been found to be Ag content dependent. An optimized value of 10 wt% Ag has been found to produce the best quality tubes showing reproducibleIcvalue > 120 Amp at 77 K which is in general a requirement to energies of the cryogen free conventional/HTSC superconducting magnets below 20 K.

The effect of thermal cycling on two-step enhanced superconducting transition in Bi1.6Pb0.4Sr2Ca1Cu2O8+x

Abstract A superconducting drop around 120 K with zero resistance temperature around 101K has been obtained in Bi 1.6 Pb 0.4 Sr 2 Ca 1 Cu 2 O 8+ x samples. Study of effect of thermal cycling on the superconducting properties has suggested that both the above enhanced transitions belong to the low- T c Bi-2212 phase.

Synthesis of Y-Ba-Cu-O superconducting material by citrate chemical route—some observations

We present an overview of our powder synthesis procedure and give some initial results concerning the superconducting properties of YBa2Cu3O7−x.