N.C. Soni

History effects in harmonic generation in sintered pellets of YBaCuO

Abstract We present measurements of harmonic generation in the magnetization of a sintered YBa2Cu3O7 pellet subject to HDC + HAC × cos cot. The magnitude of the harmonics depend strongly on whether the pellet was cooled to 77K in HDC, or whether HDC was applied after zero-field-cooling. In the latter case, we also observe a large hysteresis between the HDC increasing and HDC decreasing cases. The results are explained qualitatively.

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

Magnetic field dependence of the harmonic generation in sintered pellets of YBaCuO: The history effects

Abstract Results are presented for a detailed study of harmonic generation in the magnetization of a sintered YBa 2 Cu 3 O 7 pellet subjected to various combinations of AC and DC magnetic field. On cooling the sample below T c in zero field, a large hysteresis is observed between H DC increasing and decreasing cases. General features of this observation can be understood in terms of the hysteretic properties of the weak-linked intergrain region mediated by trapped flux inside the grains. However, there remain some finer aspects which do not have a simple explanation within this model.

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.

Study of minor hysteresis loops of YBaCuO: dc and ac hysteresis effects in granular samples

Abstract Results are presented for a detailed study of ac minor hysteresis loops in sintered pellets of YBa 2 Cu 3 O 7 subjected to various combinations of ac and dc magnetic fields. It is shown explicitly, that the history of application of B dc drastically affects the shape of the minor hysteresis loop. Also it is shown for the first time, that the shape of these loops is a function of the history of application of the cycling field B ac . A model, based on the two-component nature of an HTSC pellet, is being put forward to explain the observed dc and ac history effects.

Effect of Sb, Sn and In doping on the superconductivity in Bi-Pb-Sr-Ca-Cu-O system

Abstract Partial substitution of Pb for Bi is known to stabilize the 2223 phase in the Bi-Sr-Ca-Cu-O system. We have investigated the effect of Sb, Sn and In doping on superconductivity in the Cu-rich and Cu-deficient compositions in the Bi-Pb-Sr-Ca-Cu-O system. Samples were prepared under varying sintering conditions. No indication of a resistive transition in the vicinity of 160K is observed. The zero resistance temperatures are found to be in 65–102K range.

Superconductivity in the 70K range in Ca-free PbSrRCuO oxide system

Abstract High temperature bulk superconductivity is well established in several families of copper oxide based systems [1–4], such as, (La,M)2CuO4, LnBa2Cu3O7-δ and (Tl,Bi)m(Ba,Sr)2Can-1CunOm+2n+2. More recently, a new family of compounds with general formula Pb2Sr2ACu3O8+δ was reported by Cava et al [5] and confirmed by Subramaniam et al [6], in which bulk superconductivity near 70K was observed mostly in systems when A was a mixture of Ln and Sr or Ca. Using a modified synthesis procedure, we have prepared Ca-free compounds of the form Pb2Sr2RCu3O8+δ (R = RareEarth) and found that superconductivity near 70K occurs in this system without Ca. So far, in this series, we have observed superconductivity in compounds with R = Y, Nd, Eu, Dy, Ho, Er, YbandLu. Under the same preparation conditions, compounds with R = Pr, Sm, Gd, TbandTm also exhibit onset of superconductivity at about 70K but zero resistance state is not achieved down to 12K.

Field and temperature dependence of intergranular hysteresis in YBa2Cu3O7

We have studied the variation of low field magnetization hysteresis in YBa2Cu3O7 as a function of the maximum magnetic field applied during a hysteresis cycle (1 G<Hmax<7.3 G) and also as a function of temperature (77 K<T<95 K). The remnant magnetization is studied as a function ofHmax andT and the measured dependences are explained using the extended critical state model. The potential of this technique as a contactless method of probing the temperature dependence ofJc is discussed.

Characterisation of UO2 powders by radioactive adsorption

Abstract It has earlier been shown that radioactive adsorption may be used to characterise the sintering behaviour of UO 2 powders prepared through calcination of ADU. The present paper reports the effect of temperature and concentration on the adsorption of Mn ++ ions on UO 2 powders of varying sinterability. The variation of the density of the adsorbed Mn ++ ions on UO 2 surface follows the classical Langmuir type isotherm. The effect of temperature on the adsorption characteristic is employed to evaluate the bond strength between Mn ++ ions and the powder surface. The bond strength is highest for the 1023 K calcined powder and decreases with rise in the decomposition temperature of the ADU. However 873 K calcined powder shows lower bond strength despite the increase in the total adsorption. The reasons for the anomalous behaviour of 873 K calcined powder are not understood. These studies show that the concentration of the adsorbed species on the surface provides a measure of the sinterability and integral activity of the powder.

Hc1 measurements using hysteresis in magnetoresistance of sintered YBaCuO

We present measurements of hysteresis in the magnetoresistance of sintered YBaCuO. In analogy with hysteresis in current density and in harmonic generation, this is attributed to trapping of flux within the grains. This feature is used to track the penetration of magnetic field inside the superconducting granular regions.