B. Gopala Krishna

Elastic anomalies in sodium-doped Bi-2212 single-phase HTSC materials

A series of sodium-doped Bi-2212 single-phase high- superconductors have been prepared by the solid state reaction method. After the usual characterization by XRD, SEM, bulk density porosity etc, ultrasonic longitudinal velocity studies were undertaken over the temperature range 80 - 300 K using the pulse transmission technique. In the temperature range 300 - 230 K, in contrast to normal solids, the velocity of Bi - Na(0) and Bi - Na(3) samples is found to decrease with decreasing temperature (elastic softening) followed by a velocity maximum at around 150 K for all the four samples, signifying the presence of lattice instabilities. Plausible explanations are given for both the elastic softening and the velocity maxima on the basis of microstructure and ordering readjustments of the oxygen atoms among the Cu - O planes of the samples.

Effect of sodium substitution on superconductivity in Bi-based (2212) system

Abstract Samples with the nominal composition Bi 2 Sr 2 Ca 1− x Na x Cu 2 O y (if with x = 0.05, 0.2, 0.4 and 0.5) were prepared by the solid state reaction method. The role of Na and the effect of various heat treatments in the 2212 composition were studied. From the X-ray diffraction data it is found that the samples annealed for longer duration have shown a single phase 2212. From the DC four-probe resistivity data, it was found that the T c (zero) varies from 79 to 89 K. It is believed that doping with an alkali element would reduce the oxygen content due to monovalency and gives an effect similar to that during heat treatment under low oxygen pressure or quenching. The presence of sodium in the samples was confirmed by electron probe microanalysis.

Superconductivity in Na-doped Bi2Sr2CaCu2O y system

Samples with the nominal composition Bi2Sr2Ca1−xNaxCu2Oy (x=0, 0·1, 0·2 and 0·3) were prepared by solid-state reaction of the individual compounds. X-ray diffraction patterns indicate that the samples have a majority 2212 phase with 2223 also being present. From the DC four-probe resistance data, we have observed that the furnace-cooled samples show metallic behaviour while the quenched samples show superconductivity up to 97 K.

High Dielectric Constant and Variable Range Hopping in Bi2Sr2YCu2Oy

The sample Bi2Sr2YCu2Oy prepared by solid state reaction method showed a single phase Bi-2212. DC resistivity measured in the temperature range 77.5–300 K showed M–I like transition. AC conductivity measurements were performed in the temperature range 80–473 K at different frequencies. The conduction is by Motts Variable Range Hopping. Various parameters pertaining to the conduction process are evaluated. The value of dielectric constant e increases with temperature and shows a very high value. For a given temperature the value is found to decrease with increase in frequency.

HIGH DIELECTRIC CONSTANT AND NONLINEAR ELECTRIC RESPONSE IN Bi2Sr2SmCu2Oy

Samples with the nominal composition Bi2Sr2SmCu2Oy were prepared by solid state reaction method. From the room temperature X-ray diffraction data, it was found that the sample is similar to the Bi-2212 structure. DC electrical resistivity was done from 80 K to 573 K and the impedance measurements were performed from 80 K to 573 K at different frequencies in the range of 10 kHz to 800 kHz. The sample Bi2Sr2SmCu2Oy has exhibited semiconducting behavior in the low temperature region (80 K to 343 K), metallic behavior in the temperature range of 343 K to 443 K and again semiconducting behavior above 443 K. The sample has exhibited the phenomenon of variable rangehopping mechanism (VRH). The physical parameters related to VRH such as localization length (a), hopping distance (R) and hopping energy (W) have been evaluated and discussed. The activation energy in the high temperature region (above 300 K) decreases with increasing frequency. Tan δ increases with increase in temperature (303 K-573 K), which is attributed to increased conductivity. The dielectric constant increases with increase in temperature. For a given temperature the value of e is found to decrease with increase in frequency.

LOW TEMPERATURE ELASTIC BEHAVIOR OF POTASSIUM-DOPED Bi-2212 SINGLE PHASE HIGH Tc SUPERCONDUCTORS

A single phase Bi-2212 and two potassium-doped Bi-2212 samples have been prepared by the solid state reaction method. After the usual characterisation, the ultrasonic longitudinal velocity measurements of all the samples over a temperature range of 80–300 K were undertaken by the pulse transmission technique. The longitudinal velocity of the undoped sample is found to vary in an anomalous way followed by a maximum at 140 K. On the other hand, in the case of the two K-doped ones, the velocity is found to vary with temperature in a normal way up to 120 K followed by a maximum at 105 and 100 K respectively. A qualitative explanation for the observed behavior in all the three cases is given.

Potassium substitution in the Bi2Sr2CaCu2Oy superconductor

Abstract Samples with the nominal composition Bi2Sr2−xKxCaCu2Oy ( with x = 0.1, 0.2, 0.3, 0.4 and 0.5 ) were prepared by the solid state reaction method. The role of potassium at the Sr-site and the effect of various heat treatments in the 2212 composition were studied. From the X-ray diffraction data it is found that the samples annealed for longer have shown a single phase 2212 up to x = 0.4 dopant level. From d.c. four probe electrical resistance data, it was found that the T c (0) varies from 76 to 93 K for different dopant levels. The diamagnetic nature of the samples was confirmed by a.c. susceptibility measurements. The phase formation of 2212 and the presence of potassium in the doped samples was confirmed by electron microprobe analysis.

TIN ADDITION TO THE (Bi, Pb) 2223 SYSTEM

The effect of the addition of Sn on the superconducting properties of the Bi1.7Pb0.3Sr2Ca2Cu3Oy system as functions of Sn concentration and heat treatment has been studied by dc electrical resistance, ac magnetic susceptibility, and X-ray diffraction. Tin addition suppresses the volume fraction of the high Tc phase. Samples with Sn > 0.1 show metallic behavior up to LNT. The formation of the Ca2PbO4 phase is promoted by Sn. This depletes the amount of Pb and Ca necessary for the formation of the 2223 phase, thus reducing the volume fraction of the 2223 phase. It is possible that at least a small fraction of tin substitutes some of the cationic sites of the starting composition. The results of the different measurements are presented.

Superconductivity at 97 K in Bi2Sr2Ca1Cu2Oy system

Abstract Samples with the nominal composition Bi 2 Sr 2 Ca 1 Cu 2 O y were prepared by the solid state reaction method. Samples were prepared by various heat treatments and are characterized by using d.c. electrical resistivity, susceptibility and X-ray diffraction methods. A sample annealed for many hours at high temperature and quenched from 400 °C into liquid nitrogen has exhibited superconductivity at 97 K. The X-ray diffraction of this sample shows a single phase nature. The thermal expansion data was used for choosing the optimum quenching temperature (400 °C).

Effect of annealing and quenching on superconductivity in Pb and Sb-doped Bi2Sr2Ca2Cu3O x

Samples of the series Bi1·9−xPbxSb0·1Sr2Ca2Cu3Oy withx=0, 0·1, 0·2, 0·3 and 0·4 were prepared by the solid-state route. The X-ray and d.c. electrical resistivity data on furnace-cooled and quenched samples are presented. Though the starting composition is 2223, the end products were multiphase with 4334 as the major phase. A superconducting transition withTc=100K was observed in the pure 2223 sample after quenching. The furnace-cooled samples were metallic, while samples withx=0·1, 0·2 and 0·3 were superconducting after quenching. The amount of the 4334 phase decreases with increasing Pb content. Quenching seems to be favourable for the formation of the 4334 phase.