K. Nanda Kishore

Effect of rare-earth Sm3+ substitution on the superconducting properties of the Bi1.7Pb0.3Sr2Ca2−xSmxCu3Oy system

Abstract The superconducting properties of the Bi1.7Pb0.3Sr2Ca2−xSmxCu3Oy system have been investigated by X-ray diffraction, electrical resistance, AC susceptibility and DC magnetization. Substitution of diluted quantities of Sm3+ for Ca2+ is found to change drastically the superconducting properties of the system, despite the weak paramagnetic nature of the Sm3+ ion. X-ray diffraction results show that the volume fraction of the high-Tc (2:2:2:3) phase decreases and that of the low-Tc (2:2:1:2) phase increases as the samarium concentration increases from x=0 to 0.05. At higher samarium concentrations, i.e. for x=0.075 and 0.1, only the low-Tc phase exists. AC susceptibility measurements (χ′ versus T) show that Tc onset does not change up to x=0.02, and then decreases down to 100 K for x=0.1. The loss peak (χ″ versus T) decreases from 108 to 104 K for x=0 to x=0.02, while for higher Sm concentrations, the χ″ peak decreases more significantly. The same trend was observed by resistivity measurements and can be explained on the basis of granular superconductivity. Magnetization measurements show that the superconducting volume (slope d M d H ) decreases with increasing samarium concentration. These results show that the Ca2+→Sm3+ substitution changes the hole carrier concentration, which in turn lowers Tc and decreases the volume fraction of the superconducting phase.

Thermoelectric power studies on the Sm substituted BPSCCO (2223) superconductors

Abstract Measurements of the thermoelectric power have been carried out on Bi 1.7 Pb 0.3 Sr 2 Ca 2−x Sm x Cu 3 O y (0.0⩽ x ⩽2.0) specimens between 77 K and 300 K. The hole-carrier concentrations were calculated from the room-temperature thermopower values and it was found to decrease with increasing Sm concentration. The temperature variation of the thermopower is similar for all the samples and has indicated a two-band conduction. The results also have suggested an increased interaction of electrons and holes with phonons at low temperatures. The thermopower has attained a maximum value just above the transition temperature and thereafter decreased with increasing temperature. The thermopower value has also undergone a sign change at a temperature which varied systematically with Sm concentration. The results are presented and discussed.

Systematics of the physical properties of compounds

A study of systematics was carried out on compounds by replacing Ca with Nd ions. The dependence of on structural parameters, oxygen content and hole carrier concentration was observed as a function of Nd concentration. X-ray diffraction patterns show the single phase (2212) nature of the samples. Lattice parameters have been calculated and the results show an increase in a-axis with a simultaneous decrease in c-axis. Oxygen content was estimated from the iodometric titration method and was shown to increase with x. The measured (0) and (onset) obtained from resistivity and susceptibility data were found to decrease with increasing Nd concentration. Vacuum annealing was found to improve . The materials exhibited superconductivity in the concentration range, and for higher concentrations of Nd they became semiconducting. In the semiconducting region, the conduction was found to be due to the 3D variable range hopping (VRH) mechanism. The initial susceptibility dM/dH relating to the superconducting volume, has shown a continuous decrease with increasing Nd concentration. The hole carrier concentration was computed from thermoelectric power data and found to decrease with increasing Nd ion concentration. The results are presented and discussed in comparison with those obtained from system.

Superconducting and normal state properties of the Bi1.7Pb0.3Sr2Ca2 − xYxCu3Oy system

Abstract The superconducting and normal state properties of the Bi 1.7 Pb 0.3 Sr 2 Ca 2 − x Y x Cu 3 O y (0.0 ≤ x ≤ 0.1) system were studied by various techniques. The structural analysis was carried out by XRD and the results have revealed the conversion of the major (2223) phase to (2212) with Y substitution. The solubility limit of Y in the BISCCO (2223) system was found to be around x = 0.02. The granular nature of superconductivity in the compounds was evident from AC susceptibility results. Samples with higher Y contents have shown a two step inductive transition indicating the presence of the two phases. The thermoelectric power measurements have shown that Y substitution decreases the hole carrier concentration both in the (2223) and (2212) phases with an increased electron phonon interaction at low temperatures. The transition temperatures measured from TEP are more than that from resistivity measurements. This may be owing to the higher percolation concentration needed to observe zero resistivity when compared with the percolation concentration for zero TEP.

Elastic and plastic behavior of lead and silver doped BiSrCaCuO superconductors

Abstract Bi1.7Pb0.3AgxSr2Ca2Cu3Oy ( x = 0.0, 0.1, 0.2, 0.3 and 0.4 ) superconducting samples have been synthesized by the matrix method. From X-ray diffraction and d.c. electrical resistivity results, it has been confirmed that silver doping does not poison the superconductivity and T c (0) varied between 95 K and 105 K. The ultrasonic compressional ( V 1 ) and shear ( V S ) velocities at room temperature were determined for all the samples by the ultrasonic pulse transmission technique. The Youngs modulus and rigidity modulus values were calculated and were found to decrease with increasing silver doping. Microhardness measurements were performed using the indentation method. The hardness decreased with increase in lead concentration and further decreased with addition of silver. The hardness values were calculated for all the samples from the elastic data and these values are in good agreement with the experimental data.

Effect of Ca2CuO3 on the formation of (2223) phase in the BiPbSrCaCuO system

Abstract The effect of excess Ca 2 CuO 3 on the superconducting properties of Bi 1.7 Pb 0.3 Sr 2 Ca 2 Cu 3 O y was investigated through X-ray diffraction (XRD), resistivity and a.c. magnetic susceptibility measurements. The XRD patterns revealed that 2223 is the major phase. The intensities of the peaks corresponding to Ca 2 CuO 3 at 2θ=35.1 and 36.3 , and those corresponding to the high T c (2223) phase increased with increasing Ca 2 CuO 3 concentration. The T c (0) values, however, decreased with increasing Ca 2 CuO 3 concentration. The real part of the a.c. susceptibility showed a two-step transition and two sets of peaks were observed in χ″ vs. T plots. The first set of peaks were approximately 10 K less than the T c (0) values, whereas the second set of peaks occured in the temperature range 40–60 K. Both sets of peaks shifted towards lower temperatures with increasing Ca 2 CuO 3 concentration. The role of Ca 2 CuO 3 in the growth of 2223 phase and the mechanism of energy loss are discussed.

Growth of a high Tc phase in the Bi1.7Pb0.3Ca2Sr2Cu3Oy system

Abstract The effect of sintering time on the Bi 1.7 Pb 0.3 Ca 2 Sr 2 Cu 3 O y sample was investigated through XRD, DC resistance, AC magnetic susceptibility, SEM and critical current density ( J c ). The XRD patterns revealed that the intensity of the low angle line at 2 θ = 4.7° which corresponds to the high T c phase, increases gradually with sintering time and reaches a maximum for nearly 200–240 hours. It is also found that the percentage of the high T c phase is around 94% and that of the low T c is approximately 6%. DC resistance results also showed an increase in T c with sintering time and reached a maximum of 110 K. The real part of the AC susceptibility showed a step which corresponds to the transition from a normal to a superconducting state which gradually shifts to higher temperatures with an increase in sintering time. The magnitude of the step, which is a measure of the Meissner signal, also increased gradually with sintering time. The χ″ vs. T plots showed that with increase in sintering time, the width at half maxima decreased and the height of the peak increased. For still longer sintering times (200–240 h) the loss peak became sharper. The J c value increased linearly with increasing sintering time. SEM studies revealed that the grain size increases and an increase in J c may therefore be due to a reduction in the number of grain boundaries and other defects such as voids.

Transport and magnetic properties of Sm-substituted BISCCO (2223) superconductors (Bi1.7Pb0.3Sr2Ca2−xSmxCu3Oy)

Abstract The transport and magnetic properties of Bi 1.7 Pb 0.3 Sr 2 Ca 2−x Sm x Cu 3 O y (0 ≤ x ≤ 2.0) have been studied. The X-ray diffraction (XRD) suggests that the (2223) phase degrades with Sm substitution and the compounds with x > 0.05 show only the (2212) phase. The (2212) phase is maintained till x=2.0. The d.c. resistivity and a.c. susceptibility measurements show that Tc onset and Tc(0)s decrease with increasing Sm content. The results can be explained on the basis of granular superconductivity. The d.c. magnetization measurements performed on the compounds reveal that with increasing Sm concentration, the superconducting volume and the lower critical field decrease. The thermoelectric power (TEP) measurements reveal that with increasing Sm content, the hole carrier concentration decreases. The XRD, d.c. resistivity, a.c. susceptibility, d.c. magnetization and TEP results are presented and compared with earlier results on Gd- and Y-substituted BISCCO (2223) system.

Effect of Y3+ substitution for Ca on the superconducting and the normal state properties of Bi(2223) system (Bi1.7Pb0.3Ca2−xYxSr2Cu3Oy)

Abstract The superconducting properties of the Bi1.7Pb0.3Ca2−xYxSr2Cu3Oy system have been investigated by X-ray diffraction, SEM, DC electrical resistivity, AC susceptibility and Thermo Electric Power measurements. With increasing Y concentration the volume fraction of the high Tc (2223) phase decreased and that of the low Tc (2212) phase increased and also Tc (0)s and Tc(onset)s have decreased. The AC susceptibility measurements have shown the granular nature of the superconductivity. The thermoelectric power measurements have shown that the charge carrier concentration decreases with increasing Y concentration.

A.c. and d.c. susceptibility studies on silver-doped BPSCCO (2223) superconductors

Abstract A.c. and d.c. susceptibility studies were undertaken on Ag-doped Bi 1.7 Pb 0.3 Ca 2 Sr 2 Cu 3 O y samples to investigate the quality of the materials and the critical current density. X-ray diffraction (XRD) and resistance measurements were also performed to determine the phase of the samples and the influence of Ag addition on T c (0) . XRD results show that the (2223) high -T c phase is retained and T c (0) varies between 103 and 107 K. These results suggest that the superconducting properties are not destroyed by Ag addition. The a.c. susceptibility results show two troughs in the χ′ vs. T curves and two peaks in the χ″ vs. T plots. The first peak is smaller, is close to the transition temperature and corresponds to the midpoint of the first trough in the χ−T curve. The second peak appears below 98 K and corresponds to the midpoint of the second trough. The positions of both sets of peaks remain almost the same. The low-temperature peaks are sharper in samples containing silver compared with the pure sample, and the sharpness increases with an increase in silver concentration. J c was calculated from d.c. susceptibility data using Beans critical state and plate-like models. J c was calculated from d.c. susceptibility data using Beans critical state and plate-like models. J c increases with silver doping. The increase in sharpness of the low-temperature loss peak with an increase in Ag concentration and critical current density suggests that Ag probably precipitates along the grain boundaries and improves the interconnectivity between the grains.