Characterization and Studies of Critical Currents in Yttrium BARIUM(2) COPPER(3) OXYGEN(7-DELTA) Superconducting Ceramics
Abstract
High temperature superconducting YBa_2 Cu_3O_{7 -delta} ceramics have been fabricated using a solid state reaction technique. Typical transition temperatures and critical current density in the superconductors are 92-93 K and 200 A/cm^2 respectively. The fabrication conditions for good quality samples have been optimized. The annealing effects of the ceramics have been studied. The influence of substitution or addition of other materials including scandium, vanadium and silver into YBa_2Cu_3O _{7-delta} ceramics have been investigated. It has been found that the addition of proper amounts of silver can increase the critical current density in the superconductors significantly with no deterioration of other superconducting properties. In this work, YBa_2Cu _3O_{7-delta} ceramics have been characterized in terms of microstructure and composition, thermodynamic properties and electrical properties. X-ray diffraction measurements show a single phase of YBa_2Cu_3 O_{7-delta} for the ceramic samples. X-ray photoelectron spectroscopy, however, shows significant amounts of impurities segregated at grain boundaries. These impurities have been found detrimental to high critical currents. The thermodynamic properties of YBa_2 Cu_3O_{7 -delta} have been studied by evaluating the heat capacity of the materials. The superconducting electronic specific heat has been derived by subtracting the lattice specific heat from the experimental data. Based on this work, the reduced energy-gap of YBa_2 Cu_3O_{7 -delta} has been determined as 2 Delta/k_{B} T_{c} = 3.6 +/- 0.4, and the thermodynamic critical magnetic field H_{c} has been evaluated as a function of temperature. These studies show that the BCS theory is more applicable to YBa_2Cu_3O _{7-delta} superconductors compared with other theories. Studies of electrical properties of YBa _2Cu_3O_ {7-delta} ceramics focus on the properties of weak links in the materials. By studying critical currents as a function of temperature, superconductor --normal conductor--superconductor (SNS) weak links have been found dominant in YBa_2Cu _3O_{7-delta} ceramics. Based on this fact, a model which is able to describe different I_{c}-T characteristics of the ceramics has been developed according to the Ginzburg-Landau theory and the de Gennes proximity effects.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT........59Y
- Keywords:
-
- YTTRIUM BARIUM COPPER OXIDE;
- BARIUM COPPER OXIDE;
- Physics: Condensed Matter; Physics: Electricity and Magnetism