Daniel Antonio Esparza

magnetic properties of glasses with high iron oxide content

A study of the magnetic properties of glasses with high iron oxide content has been carried out. This glass series was obtained by recycling goethite (FeOOH) industrial waste, with dolomite and glass cullet as complementary raw materials. The magnetisation as a function of applied magnetic field for these glasses was measured. According to the magnetic behaviour, which closely correlates with the iron content, these glasses can be classified as ferrimagnetic, superparamagnetic and paramagnetic. The magnetisation curves were analysed in terms of current models for these types of magnetic behaviour.

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.

The sintering kinetics of Ag-Y1Ba2Cu3O7−δ laminates and the interfacial metal-ceramic reaction☆

Abstract The tape casting technique was successfully applied for making Ag-Y 1 Ba 2 Cu 3 O 7−δ laminates having alternative layers of Ag and Y 1 Ba 2 Cu 3 O 7−δ (YBC), forming a YBC/Ag/YBC/Ag/ YBC multilayer composite. For the Ag-YBC composite two well differentiated temperature ranges for densification were observed. A low temperature range from 200 to 800 °C, where the principal densification could be assigned to the silver part of the structure, and a high temperature range where the main densification features of YBC were still observed for the Ag-YBC laminates. Silver provoked a noticeable decrease in the “on-set” temperatures for each one of the regimes operating in the densification of YBC. From kinetic analysis, it may be advanced that silver alters significantly the diffusivities in the YBC ceramic system. For temperatures in the range 500–600 °C, the Ag and the YBC ceramic were found to react to a significant extent causing decomposition of YBC close to the interface. At the interface on the YBC ceramic side, we observed BaCuO 2 and CuO grains containing Ag. After treatment we observed an interface on the Ag side with silver grains much smaller in size than those in the bulk of the Ag layer.

Influence of Sr-content in YBa2−xSrxCu3Oy formation and superconductivity

Abstract DTA analysis of YBa 2−x Sr X Cu 3 O y showed that the temperature of the peritectic peak increases with Sr-content. Thermogravimetric measurements showed that the oxygen non-stoichiometric range between 900 °C and room temperature decreases as the Sr-content increases. From these results, it is proposed that the Sr stabilizes the 1:2:3 phase, given rise to a doped structure with a larger cohesive energy.

Influence of the sample geometry on critical current density of high Tc granular superconductors

I‐V characteristics of a polycrystalline Y‐Ba‐Cu‐O sample of rectangular cross section were measured while varying the specimen thickness by successive polishings. The critical current density (Jc) was found to increase with the reduction of the sample thickness (e). A model based on junctions decoupling due to the self‐magnetic field generated by the transport current is proposed to explain our results. For this particular geometry and assuming a uniform current distribution along the sample, the magnetic field distribution inside the sample was calculated. From this, an expression for Jc as a function of e was derived which is in good agreement with the experimental results. From the measured values, the value of the decoupling field obtained was of 4.47 Oe.

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.

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.

BaCuO2 - Ag interaction: a central feature in the densification of Ag - Y1Ba2Cu3O7-δ composites

Abstract Dilatometric and DTA / TGA measurements (in O 2 atmosphere) on Ag-123 composites indicate that silver affects dramatically the two principal reactions that provoke the unstabilization of Y 1 Ba 2 Cu 3 O 7-δ . These are the precipitation of 011 phase plus Yttrium rich liquid (900–930°C), and the peritectic reaction triggered by 011 fusion (980–1040°C). Silver increases, upon initial heating, the partial decomposition of 123 phase (ER). For pre-peritectic temperatures higher than the AgAgO eutectic temperature, it interacts with 011 phase for developing a “re-assembling mechanism”. A modified 123 phase named 123 ∗ is formed, containing doping levels of Ag, possibly as a layer of 123 ∗ around 123 grains. SEM / EDAX analysis show the superficial nature of the re-assembling phenomena.

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.