A. Memon

THE INFLUENCE OF Pb, Ni AND V DOPING ON THE CHARACTERISTICS OF HIGH-TcBi-Si-Ca-Cu-O SUPERCONDUCTORS

We report on the studies of ac susceptibility, ultrasonic attenuation, superconductivity and some calorimetric measurements on Pb-doped for Bi-site as well as Ni and V substitution for Cu in Bi-Sr-Ca-Cu-O superconductors. It was confirmed that the Bi(Pb)-based glass ceramics (i.e. crystallized glasses) exhibit superconductivity with Tc above 105 K higher than Bi(Ni) with Tc 68 K and Bi(V)-based glasses with Tc 40K. X-ray diffraction analysis revealed that the structure of these materials is Bi-2223 type with a=4.067 A and C=30.768 A. It was found from the temperature dependence of ac complex susceptibility that the coupling nature at grain boundaries of superconducting crystals in the samples obtained was weak, and was improved by increasing the annealing time. Data are discussed for varying Tc of different substituents. Our results on the ultrasonic attenuation revealed a series of relaxation peaks with low activation enthalpies which indicate that electronic defects may be the cause for the relaxation processes.

Cation distribution investigation of the system Li0.5Fe x Ga2.50-xO4 by X-rays, electrical conductivity and Mossbauer studies

AbstractStructural, electrical and Mossbauer studies were carried out for the system Li0.5FexGa2.50-xO4. All the compounds with 0 ⩽ x⩽ 2.5 crystallised with cubic spinel structure. Lattice constant values calculated from XRD analysis were found to increase with increasing x. X-ray intensity calculations indicated that Li1+ occupies only the octahedral site and Ga3+ and Fe3+ ions occupy both octahedral and tetrahedral sites. Activation energy and thermoelectric coefficient values decreased with increasing values of x. All the compounds studied were p-type semiconductors and possess low mobility values of 10−7-10−9 cm2V−1 s−1. Mossbauer data show the presence of iron in the Fe3+ state and the isomer shift values for all the compositions of the system are within the range of high spin ferric compounds. The probable ionic configuration for the system is suggested as:

Superconducting properties of glass ceramics in the Bi-R-Sr-Ca-Cu-O (with R=Pb or Ag) system

{\text{Ga}}_{{\text{1 - }}\alpha }^{{\text{3 + }}} {\text{Fe}}_\alpha ^{{\text{3 + }}} [Li_{0.5}^1 {\text{Fe}}_{{\text{x - }}\alpha }^{{\text{3 + }}} {\text{Ga}}_{{\text{2}}{\text{.5 - x + }}\alpha }^{\text{3}} ] {\text{O}}_{\text{4}}^{{\text{2 - }}}

The mechanical and structural properties of Y1Ba2Cu3O7−x superconductors

The optical properties of vanadium-phosphate glasses containing various amounts of (V2O5)50(P2O5)50−x(VCl3)x, wherex = 0.05, 0.06, 0.08 and 0.1, were measured as a function of VCl3 content. It is found that by adding VCl3 to the melt when the glass is formed, the added chlorine which acts as an oxidizing agent alters the ratio of concentration of vanadium ions and thus the absorption coefficient. Data on absorption in this study show that the power law,α(ω) =B(ħω) −E0)n/ħω is best followed forn = 3, and the absorption is solely due to vanadium oxide. Furthermore, the infrared absorption spectra of this system were investigated in the wave number range 400 to 4600cm−1. The addition of VCl3 to the vanadium-phosphate glasses does not seem to introduce any new absorption band in this range as compared with the spectrum of a pure vanadium-phosphate glass. Furthermore, no significant difference in the absorption spectra was observed by annealing the V2O5-P2O5 glasses up to 200° C.

Structural, electrical and spectroscopic studies of the system Ni1-xCuxFe2O4

The influence of partial substitution of Pb and Ag in the Bi1-xRxSrCaCu1.5Oy (R=Pb or Ag; x=0.2) glass ceramics has been investigated. It was found that the Ag-containing compound exhibited superconductivity with a Tc of 78 K while the Pb-containing compound exhibited a Tc of 87 K. The ultrasonic attenuation was measured at frequencies of 5, 10, 20 and 30 MHz in the temperature range 77 to 300 K. Five relaxation peaks were observed in Bi0.8Pb0.2SrCaCu1.5Oy and Bi0.8Ag0.2SrCaCu1.5Oy compounds. As the frequency of the measurement is increased, the temperature of the peaks shifts to higher values while the height of the peaks decreases. The first three peaks (P1, P2 and P3) may be attributed to the electronic defects (hopping of holes) and the other two could be ascribed to the thermally activated movement of boundaries by interaction with the defects.

Structural and superconducting properties of vanadium substituted Y Ba2 (Cu1−x Vx)3 O9 perovskite oxide system

Abstract High quality samples of 110 K(Bi 1.6 Pb 0.4 ) Sr 1.7 Ca 2.3 Cu 3 O y were prepared in the form of long-length rods by rapid quenching of the melt, which showed a glass transition temperature of 379.77°C, crystallization temperature of 463.4°C and melting temperature of 861.5°C. We also prepared glasses of Bi 1-x R x SrCaCuO y (R = Al and x = 0.5) system at various melting temperatures which on further heat treatment under appropriate conditions, crystallized into superconducting phase(s). It was found that copper ions in the glasses exists mainly in the Cu(I) ionic state and the Cu(I)/Cu(total) ratio increases with glass melting temperature. The control of the Cu(I)/Cu(total) ratio leads to the successful continuous drawing of BiCaSrCuO glass fibers.

Strength of infrared phonons in R Ba2Cu3O7-x ceramic superconductors

Abstract We report on the investigation of infrared reflectance spectroscopy on Bi2Sr2CaCu2O8 glass ceramics superconductor in the temperature range 20 K to 300 K and between the frequency of 100 to 20000cm−1. The frequency dependent conductivity was calculated by Kramer-Kronig transformation of the reflectance spectra. Analysis by Drude and lattice vibration models reveals the Drude plasma frequency and the reflectance edge, closer to those of a single crystal. All the predicted phonon modes are observed in the measured spectra. We did not find any appreciable change in the frequency and oscillator strength of the phonons in the present temperature range.