Maria Teresa Malachevsky

SEM investigation of the critical current degradation onset in BSCCO-2223 multifilamentary tapes submitted to bending

The applicability of superconducting tapes is evaluated employing the definition of an irreversibility strain irr, attained at a critical current density Jc of 98% of its initial value. Before reaching this limit, the Jc decreases slowly. In order to determine the microstructural changes responsible for this behaviour, we investigated the mechanical response of 19-filament BSSCCO-Ag tapes with different ceramic microstructures, when bent without exceeding irr. After chemically removing a rectangular-shaped area of the silver sheath to expose the ceramic filaments, the samples were subjected to three-point bending inside an SEM chamber. Thus, we were able to follow the microstructural changes produced by the formation and propagation of cracks at the same time that we recorded on a PC the force versus deflection curves. We identified three different regions representing the response of the tapes to increasing bending. After the elastic region, the pre-existing microstructural faults determined the plastic behaviour. At higher deflections, we measured several stress drops coincident with the observation of micro-cracks that grew slowly across the filaments. Twin samples were employed to control the critical current density decay as a function of the bending deflection introduced at ambient temperature. We investigated the correlation between the bending induced defects and the critical current degradation.

Metal–ceramics interface response to bending and fatigue cycles in superconducting Ag-Bi2223 composites

Abstract Bending cycles in superconducting composites lead to degradation of their transport properties. We had studied this effect on Ag-Bi2223 composites, in the form of tapes with 1, 7 and 19 ceramic filaments. The samples were subjected to increasing bending–straightening strains, measuring the critical current density Jc at 77 K, 0 T after each cycle, in order to determine the maximum allowed strain (emax) before degradation of the Jc occurs. Fatigue tests were performed by cycling the tapes on bending to a fixed radius. For this purpose, two mandrels with different diameters were selected: one resulting in strains e emax. The behavior of Jc as a function of the number of cycles was observed for the three composites, indicating that multifilamentary tapes withstand cycling better. As the transport properties in composite superconductors are mainly characterized by their metal–ceramic interface, the silver sheath was chemically removed after the last cycle to investigate the interfacial microstructure. The exposed ceramics was observed with SEM to identify the crack formation mechanism: mainly intergranular cracks in the face subjected to tension and transgranular cracks in the face subjected to compression.

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.

Bending strain tolerance of BPSCCO-2223 tapes

The degradation of critical current density (Jc) at 77 K in superconducting tapes was evaluated for combined tension-compression strain. Tapes were prepared by pressing or rolling BPSCCO-2223 Ag-sheathed wires. Lengths of approximately 8 cm were bent by winding on cylinders of different diameters (between 15 to 0.8 cm) at room temperature. The Jc at 77 K was measured either after letting the tape to relax or with the applied strain. The bending strain was calculated as the ratio between the total thickness of the tape (silver sheath plus ceramic core) and the curvature diameter. The strain tolerance for each case was obtained from the deformation Vs Jc characteristics. We present the analysis of the results along with the microstructure, thickness, preparation procedure of the tapes and the degradation of Jc with thermal cycling.

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.

Metal–ceramic interfaces in Ag-BSCCO composites

Abstract The texture and formation of the superconductor at Ag-BSCCO interface was investigated. Thick films and silver-sheathed tapes were employed for this purpose. The metal, the ceramic in contact with the metal and the ceramic surfaces were analyzed separately. X-ray diffraction experiments and SEM observations revealed the degree of texture in the superconductor as a function of the distance from the silver. The grain alignment is more pronounced at the zone in contact with the silver sheath, suggesting the existence of a cooperative effect between the silver and the superconductor. We propose that silver textures forming a rectangular lattice that acts as habit plane for the growth of the oriented ceramic grains. The sintering atmosphere plays a crucial role in the formation of the superconducting phases.