Claudio Alberto D'Ovidio

The granular nature of bulk superconductivity at 40K in La1.8Sr0.2CuO4

Abstract We have measured the electrical resistivity, magnetic flux expulsion and critical fields of the new high temperature superconductor La 1.8 Sr 0.2 CuO 4 after different heat treatments. Our data show that the complete flux expulsion in these samples is consistent with the presence of shielding currents induced in multiple connected structures. The magnetization measurements indicate that the lowest field where flux penetrates into the sample should not be taken to be the bulk Hcl. It is shown that the absolute value of the electrical resistivity and its temperature dependence are not correlated with the superconducting critical field and temperature. We discuss the behaviour of the superconducting parameters and their relation with the electrical resistivity.

Non linear temperature dependence of the resistivity of LaSrCuO: Effects of Sr content

Abstract The linear temperature dependence of the electrical resistivity has been taken to be one of the characteristic features of the ceramic superconductors. Measurements of the electrical resistance of the LaSrCuO system in a wide temperature range show that the previous statement is only an approximation valid for some values of the Sr concentration.

The nature of the magnetic flux behavior of the superconductor LaSrcuO

Abstract Precise magnetization measurements in La1.85Sr.15CuO4 from 1.5K to 40K and from 1 mOe to 300 Oe show that the magnetic flux penetration can be understood in terms of a simple critical state model, applied to a multiple connected non homogeneous superconductor. The magnetization data together with the results from vibrating reed experiments show that the magnetic flux penetration at the lowest fields is not associated with superconducting vortices. The vortices penetrate at a field Hcl characteristic of the regions where the order parameter has its maximum value.

Cubic Phases in the Gd2O3-ZrO2 and Dy2O3-TiO2 Systems for Nuclear Industry Applications

Neutron absorbers are elements with a high neutron capture cross section that are employed at nuclear reactors to control excess fuel reactivity. If these absorbers are converted into materials of relatively low absorption cross section as the result of neutron absorption, they consume during the reactor core life and so are called burnable. These elements can be distributed inside an oxide ceramic that is stable under irradiation and thus called inert. Cubic zirconium oxide is one of the preferred materials to be used as inert matrix. It is stable under irradiation, experiments very low swelling, and is isomorphic to uranium oxide. The cubic phase is stabilized by adding small amounts of dopants like Dy2O3 and Gd2O3. As both dysprosium and gadolinium have a high neutron cross section, they are good candidates to prepare burnable neutron absorbers. Pyrochlores, like Gd2Zr2O7 and Dy2Ti2O7, allow the solid solution of a large quantity of elements besides being stable under irradiation. These characteristics make them also useful for safe storage of nuclear wastes. We present a preliminary study of the thermal analysis of different compositions in the systems Gd2O3-ZrO2 and Dy2O3-TiO2, investigating the feasibility to obtain oxide ceramics useful for the nuclear industry.

In situ evaluation of the interfacial crack evolution in Ag-BSCCO tapes subjected to three-point bending

Abstract When a one-filament Ag-BSCCO tape is subjected to bending, the ceramic in contact with the metal sheath of the outer face would suffer the maximum tension loads. Crack formation will start at this point, leading to a decrease in the critical current density J c . The study of the crack evolution of the ceramic at the interface can be used as a factor of merit for the composite performance. After chemically removing a square-shaped area of the silver sheath, we subjected the samples to in situ three-point bending inside a SEM chamber, directly following the microstructural changes produced by the formation and propagation of cracks. The influence of the intermediate deformation procedures employed for the tape fabrication on the mechanical and transport properties was also investigated.

Fast device for large transport critical current measurements

Abstract In this work we describe some modifications that improve an equipment for the fast measurement of transport critical currents in superconducting materials having a critical current of hundreds of amperes. It is based on the appliance of an electrical current for a very short period of time, rapid enough to preserve the integrity of the current leads and minimize the Joule effect. Power is applied to the wire-sample ensemble and the voltage drop is measured within seconds, with a resolution that can reach the order of 10 nV. The hardware is composed of three parts: the current pulse generator, a low noise-quick response differential voltage amplifier and a PC with a digital to analog converter—analog to digital converter card. The data acquisition is achieved via an Assembler program.

Thermal conductivity and grain size of ceramic La1.8Sr0.2CuO4

Abstract In order to study the important of Casimir phonon scattering on the thermal conductivity of La1.8Sr0.2CuO4, we measure the thermal conductivity between 1 and 30 K on samples of different grain sizes. We conclude that it starts to be important in this temperature range for grain sizes d

Superconductivity in the BiSrCaCuO system

Abstract We have prepared samples with different compositions and heat treatments, replacing part of the bismuth by other heavy elements. By means of x-ray diffractometry we have analyzed the presence of the different phases formed in each case. We report the resistivity and inductive measurements and discuss the influence of the heat treatments and compositions in the quantity and stability of the phases.

Setup for fast-pulsed measurements of large critical currents

Abstract We describe a set of equipments for pulsed measurements of transport critical currents in superconducting materials having a critical current of tens or hundreds of amperes. It is based on the appliance of an electrical current for a very short period of time, rapid enough to preserve the integrity of the current leads and to minimize the Joule effect. Power is applied to the wire–sample setup and the voltage drop is measured within seconds, with a resolution of the order of 10 nV. In this way the I – V characteristics can be obtained with a 1% error, if the 1 μV / cm criterion is employed. The hardware is composed of three parts: the current pulse generator, a fast low-noise voltage amplifier and a PC with a DAC–ADC card. The data acquisition is achieved via an Assembler program.