Ashok Mohan

Composites of superconducting YBa2Cu3O7−y with silver

Abstract In order to achieve better fabricability of the otherwise brittle ceramic superconductor YBa2Cu3O7−y, we have formed composites of this superconductor with metals such as Ag, Cu and Al. It is found that up to 50 wt.% of silver can be added to YBa2Cu3O7−ywithout degradation of its superconducting transition temperature. Such composites, sintered at about 850°C, can be easily rolled in the form of strips. Superconductivity with nearly the same Tc as in the parent compound is retained in the composites even after cold working, thus obviating the need to regenerate superconductivity by heat treatment. The superconducting properties of YBa2Cu3O7−y, however, degrade when composites are formed with Cu and Al.

Superconductivity in YBa2(Cu1−xAgx)3O7−δ oxide system

Abstract The oxide YBa2Cu3O7−δ exhibits superconductivity in the liquid nitrogen temperature range. The Cu ion is in a mixed-valent state in this system and is thought to play a very important role in the occurrence of superconductivity. We have studied the effect of partial replacement of mixed-valent Cu by monovalent Ag on the superconductivity in this system. Although normal state magnetic susceptibility measurements indicate an increase in the Cu3+/Cu2+ ratio on Ag substitution, the superconducting transition temperature is observed to decreases. Creation of oxygen vacancies in the presence of Ag may be partly responsible for the reduction in Tc

Sintering diagrams of UO2

Abstract Ashbys method of constructing sintering diagrams has been modified to obtain relative contribution diagrams directly from the computer. It is endeavoured here to study the interplay of sintering variables and mechanisms and determine the factors that affect the participation of mechanisms in UO 2 . By studying the physical properties, it emerges that the order of inaccuracies is small in most cases and do not affect the diagrams. On the other hand, even a 10% error in activation energies, which is quite plausible, would make a significant difference to the diagram. The main criticism of Ashbys approach is that the numerous properties and equations used, communicate their inaccuracies to the diagrams and make them unreliable. Our study has considerably reduced the number of factors that need to be refined to make the sintering diagrams more meaningful.

Magnetic studies on the high temperature oxide superconductor Eu0.4Gd0.6Ba2Cu3O7−δ

Abstract The oxide Eu0.4Gd0.6Ba2Cu3O7−δ is superconducting with a transition temperature above liquid nitrogen temperature. We have carried out detailed magnetic studies on this compound. Low field dc susceptibility measurements and four probe electrical resistivity measurements yield a superconducting transition temperature of about 78 K. Measurements of the shielding effect give a value of 82% of the corresponding value for perfect diamagnetism. The lower critical field is found to be 600 Oe at 4.2 K. The hysteresis loop has been obtained at 4.2 K in fields up to 90 kOe from which the magnetic critical current density has been estimated. Above the superconducting transition temperature, the magnetic susceptibility of this compound is close to that of an assemblage of Gd3+ ions. In the superconducting state it is observed that, because of field penetration, the paramagnetism of the Gd3+ ion overtakes the diamagnetism of the material in an external field of about 5 kOe.

Lower critical field and magnetization critical current density in the oxide superconductor TmBa2Cu3O7−y

Abstract We have carried out detailed magnetic measurements on the oxide superconductor TmBa 2 Cu 3 O 7− y at various temperatures and applied fields. From the hysteresis loop obtained at 5 K and the temperature dependence of the remanent magnetization, values of the lower critical field ( H c1 ), magnetization critical current density and its temperature dependence are determined. Field penetration in the superconducting state occurs for fields higher than H c1 which results in a paramagnetic contribution from the Tm 3+ ions. Above the superconducting transition temperature ( T c ), the susceptibility follows Curie-Weiss behavior with an effective moment close to the value of the free Tm 3+ ion. The strong magnetism of the rare earth ion does not very much influence the T c .

Consolidation of YBaCuO powders by shock loading

Superconducting oxide-copper monoliths have been fabricated by shock-wave loading using cylindrical and plane geometries. Bulk densities up to 96% T.D. have been obtained by varying detonation parameters. The superconducting properties, interface bonding and microstructure of the compacts have been evaluated.

Magnetic studies on the superconducting oxide ErBa2Cu3O7−y

Abstract Magnetic studies have been carried out on the oxide superconductor ErBa2Cu3O7−y (Tc=88 K) at various temperatures and fields. From the hysteresis loop recorded at 4.2K, the values of the lower critical field and magnetization critical current densities are determined. In the superconducting state, the magnetization is found to become positive at some temperature depending on the applied field. This is thought to be due to some field penetration under the influence of which the paramagnetism of Er ions overtakes the diamagnetic response. From the susceptibility data above the superconducting state, the effective rare earth ion moment is found to be very close to its free ion value. In spite of the large moment, the rare earth ion has no discernible effect on Tc.

YBa2Cu3O7−y composites for improved fabricability

Composites of YBa2Cu3O7−y with silver powder have been investigated for their fabricability and superconductivity. Silver does not react with the oxide powder. The composites can be easily rolled into thin strips. There is no appreciable degradation inTc either during sintering or rolling.

Resistivity and magnetic susceptibility of superconducting and non-superconducting ErBa2Cu3O7−y

Abstract The oxide ErBa 2 Cu 3 O 7− y has been obtained in the orthorhombic and the tetragonal forms by suitable heat treatments. The orthorhombic form is metallic and superconducting with T c =88 K while the tetragonal form has high resistance and is non-superconducting. Magnetic studies on both types of compounds reveal that the rare earth ion moment is very close to its free ion value independent of the structure. In the tetragonal compound, deviations in the susceptibility from the Curie-Weiss behavior are noted at low temperatures which may be due to the effect of crystalline electric fields.

Superconductivity in the liquid-nitrogen temperature range in the YBa&.zsbnd;CuO system

Abstract Oxides with nominal metal ion composition (Y 1-x Ba x ) 2 CuO 4 have been prepared by the solid state reaction of Y 2 O 3 , BaCO 3 and CuO for x values ranging between 0.2 and 0.5. All these are are found to be multiphase materials. Four probe resistivity measurements and low field susceptibility measurements reveal that all the samples mentioned above exhibit superconductivity at high temperatures. The mid point of the superconducting transition and the transition width are found to depend on the Barium concentration. However, the onset temperature is about 90K and is nearly the same in all these compounds.