D. Bhattacharya

Pyrophoric synthesis technique for multicomponent high‐temperature superconductors

A pyrophoric technique using an explosive exothermic reaction has been developed to yield calcined and microscopically homogeneous (0.3 μm size) powders of multicomponent cuprate systems. The technique has been used to synthesize superconducting materials of variable composition of the type M1M2⋅⋅⋅MnOx. The bulk materials prepared from these powders show sharp (1 K) superconducting transitions and nearly ideal Meissner effect.

Sintering studies on submicrometre-sized Y-Ba-Cu-oxide powder

The isothermal sintering behaviour of submicrometre-sized (<50 nm) powders of single-phase YBa2Cu3Ox (123) and unreacted stoichiometric mixture of submicrometre-sized (<50 nm) powders of BaCO3, Y2O3 and CuO (which on calcination at 1173 K gives YBa2Cu3Ox) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies,Q, of sintering were determined to be 1218 kJ mol−1 in argon, 1593 kJ mor−1 in air and 2142 kJ mol−1 in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energyQ=627kJ mol−1, and a second one above 1173 K withQ=383.7 kJ mol−1. A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.

Excess electrical conductivity and thermoelectric power of (YBa2Cu3.05Ox)Agn pellets

Temperature dependence of electrical resistivity and thermoelectric power of (YBa2Cu3.05Ox)Agn pellets are reported. Data on electrical conductivity near T >Tc are studied in terms of AL formulations. The lower limit of G.L. breakdown temperature appears to be strongly dependent on silver concentration. Apart from usual 2D and 3D regions, with increase of silver content in Yue5f8Baue5f8Cuue5f8O a third region close to Tc is revealed having critical exponent = −0.28. Thermoelectric power was found to follow a relation of the type S = AT + BT. Analysis of our data indicates that the TEP precursor peak near T >Tc arises due to superconducting fluctuations.

Lithographic patterning of superconducting YBCO films

Microbridges of YBa2Cu3O7 thin films have been fabricated by conventional photolithography and wet chemical etching using EDTA, and by the lift-off lithography technique. The variation of etch rate with etch time, etchant temperature, and post-deposition sintering temperature has been studied. It has been shown that both techniques are useful for film patterning. However, an additional sintering step is necessary for the chemically etched sample to regain the original film properties. An order of increase in critical current density is observed for the patterned film.

Thermoelectric power of undoped and doped Y-Ba-Cu-O superconductor between 77 and 300K

Temperature dependence of electrical resistivity and thermoelectric power of undoped Y-Ba-Cu-O sintered pellet and that doped with small amount of sulphur and silver are presented. While the nature of the temperature dependence of electrical resistivity and thermoelectric power remains similar, their absolute values are strongly influenced by the presence of the dopant. Thermoelectric power data for all the samples can be fitted to an expression of the type, S = AT +B/T. It is argued from the data that the precursor peak in thermoelectric power near T > Tc is of electronic origin due to superconducting fluctuations.

Superconductivity in sulphur doped YBaCuO system

Sulphur substituted YBa2Cu3SO6-x superconducting powder was prepared by the ceramic route. The material, in bulk, showed zero resistivity at 88 K and the onset of diamagnetism at 90 K. Magnetisation behaviour confirms type II behaviour of the superconducting state. The ESCA and IR spectrum on the superconducting compound at room temperature, indicate the location of the sulphur at the apex of the octahedral coordination sphere of the Cu(2)-ion in the crystal lattice.

Critical current density and magnetic behaviour of superconducting YBa2Cu3+yO7- delta : Agx composite

The effects of silver additive on the superconducting properties of YBa2Cu3+yO7- delta :Agx for x=0.1, 0.3, 0.6 and 1.2 are studied by measuring the real and imaginary components of the magnetic response in an AC field. The data are fitted to Beans critical state model and are used to obtain the macroscopic critical current density. The magnetic susceptibility and critical current density are found to depend on the silver content whereas Tc remains essentially unchanged. For x=1.2 the critical current density sharply increases below 77 K. The results suggest that the silver improves the properties of grain boundaries and enhances the intergranular coupling. The real part of the magnetic response is found to be a function of Ha/H*, where H* is the value of the field when the critical-state profile reaches the centre of the specimen.

Fabrication and characterization of Y-Ba-Cu-Oxide whisker

Y-Ba-Cu-oxide (YBCO) whiskers have been synthesised and characterised. Mass transport either by gas evolution or liquid phase is the cause for observed acicular growth. YBCO whiskers reinforced pellets show partial c-axis orientation and exhibit sharp superconducting transition at 92.4 K with transition width of 1 K. Inter granular critical current density (J s ) has been estimated from the ac susceptibility measurements by using critical state model.

Electromagnetic response of superconducting YBaCuO-Ag composite in the r.f. region

Abstract The surface resistance and r.f. screening length of YBa 2 Cu 3 O 7−x -Ag 0.6 ceramic composite are measured at 1 MHz from 110 K to 77 K. The surface resistance exhibits a peak just below T c and then decreases sharply as T is lowered. Its temperature dependence can be described by the two-fluid model. The decrease of the screening length for T c also follows the two-fluid models prediction but its value at T = 0 is found to be more than one order of magnitude higher than the reported London-penetration depth in YBaCuO superconductor.

Degradation of BPSCCO superconductors during processing

Evaporation losses of Bi, Sr, Cu and Pb during processing of Bi-Pb-Sr-Ca-Cu-oxide (BPSCCO) superconductors were observed, which affected the superconducting properties. Incongruent loss of Bi, Sr, Cu and Pb was reduced by covering the pellets by excess BPSCCO powder during sintering and formation of the 2223 phase was observed to be enhanced. Formation of low temperature eutectic by silver addition was observed to accelerate the rate of evaporation. Lower temperature processing was required for fabrication of high quality Ag sheathed BPSCCO tapes.