Upadhyayula V Varadaraju

The oxygen-deficient phase of the high T c superconductor, YBa2Cu3O7, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa2Cu3O7 powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0-90, 90-180 and 180-420 μm. The fluidized particles were crystalline with orthorhombic distortion. T c onset, estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7-85·3%, one of the highest values reported so far for YBa2Cu3O7. © 1989 Indian Academy of Sciences.

Resistance and thermopower measurements have been made on a series of compounds, YBa2Cu3-xZnx07_Yl, with x - 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below TM, the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et ai From the plot of logarithm of resistivity vs. (TM- T)1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of A, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above Tc. This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with Tc, and 2Δ/kTchas an approximate value of 5.5. © 1989, Taylor & Francis Group, LLC. All rights reserved.

Structure and T//c data on LnBa//2Cu//3O//7 (Ln equals Dy, Ho, Er, Tm, Yb and Y) compounds are reported and discussed. No reentrant behavior has been noted in these high-T//c superconductors for Ln equals Er and Yb until 4. 2 K. Four-probe resistivity was measured in the range 77-300 K employing ultrasonically soldered In-contacts. An inductance bridge with carrier-frequency amplifier was used to quickly check the samples for superconductivity at and above 77 K. A calibrated test set-up was employed to measure the superconductivity transition by the magnetic susceptibility technique.

The specific heat of samples of Y1-xPrxBa2Cu3O7-y for x = 0.1 to 0.5 has been measured from 4.5 to 65 K using a quasi-adiabatic calorimeter. A plot of C/T vs T2 shows a linear behavior below 20 K. The excess specific heat due to praseodymium in all the compounds is obtained by subtracting the specific heat of YBa2Cu3O7-x from the that of doped compound. The excess specific heat shows a peak near 19 K and its height decreases with increase in x. The excess specific heat could be interpreted on the basis of Schottky anomalies due to Pr3+ and Pr4+ ions, the peak at 19 K arising from the former. The fraction of Pr3+ ion obtained from a fit to the specific heat curves decreases from 0.30 for x = 0.1 to 0.05 for x = 0.5. © 1988.

The effect of oxide additives-CuO, SiO2, Y2O3, Bi2O3 and ZnO in 1-10 mol% on the sintering and superconducting properties of YBa2Cu3O7 was studied. SEM studies indicated improvement of grain size and interconnectivity due to the additives, the best results being obtained with Bi2O3, SiO2 and Y2O3. The superconducting transition temperature is unaffected (92±2 K) even with 10 mol % of the additives. ZnO, however, decreases the T c as expected. © 1989 Indian Academy of Sciences.

Phases of the type LnA2Nb3Oy, Ln=La, Y; A=Sr,Ba with Nb in +2 and/or +4 oxidation state have been synthesized and studied in order to explore the possibility of high Tc superconductivity. Preliminary tests showed no superconductivity above 77K. © 1988.

Studies on the superconductivity behaviour of pure and doped (Pb, Tl) Bi 4 Sr 3 Ca 3 Cu 4 O 16 phases are reported. Effect of oxide additives PbO, BaPbO 3 and BaBiO 3 has also been examined. Results show that an oxygen treatment at 600°C for 12 h during the final stages of the synthesis has the beneficial effect of sharpening the T c zero of the 4334 Bi-composition. Pb and T1 substitution has only a marginally beneficial effect on the T c, though it is not definitely known whether substitution is actually occurring in the system. PbO as an additive raises the T c zero to 89 K, whereas other additives do not basically affect the properties of the 4334 system. © 1989, Taylor & Francis Group, LLC. All rights reserved.

Studies on the title compounds show that while La and Pr form complete solid solutions, Tb does not substitute for Y. Superconducting transition temperature (Tc) decreased with increasing x from 91K but pronounced effect is noted for Ln=Pr. PrBa2Cu3O7 exhibits a semiconductor behavior. © 1988 Elsevier Science Publishers B.V.

Thick films of the high T csuperconducting oxides, LnBa2Cu3O7, Ln = Eu, Y, have been fabricated by screen printing on alumina and SrTiO3 substrates. Conditions for optimum superconductivity behaviour of the films have been established. T c onset varies from 90-94 K for all the films but zero resistance was observed only in a few cases. © 1988 Indian Academy of Sciences.

The absolute thermopower of single phase YBa2Cu3O7 and Y0.8Er0.2Ba2Cu3O7 has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down to T c. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms. © 1987 Indian Academy of Sciences.