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摘要
The main scope of this comprehensive study is to determine the critical dopant level of metal to insulator transition (MIT) for Bi-site La substituted Bi-2212 superconducting ceramics by means of bulk density, Xray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), dc resistivity (rho-T) and transport critical current density (J(c)) experimental methods. The samples produced in this work are prepared by the conventional solid state reaction technique at the constant temperature of 840 degrees C for the annealing duration of 60 h. All the experimental evidences obtained indicate that the microstructural, electrical and superconducting properties suppress with the increment of the La dopant level in the Bi-2212 superconducting matrix up to x = 0.20 beyond which the crucial characteristics diminish rapidly due to not only the deformation between the Bi-2212 layers and grain boundaries but also the disorder and hole localization (filling) problem in the crystal structure. As for the numerical values, onset and offset critical temperatures systematically decrease from 85.67 K and 84.49 K to 82.74 K and 59.38 K with the impurity content until x = 0.15. However, the sample doped with the content level of x = 0.20 exhibits further pseudo-critical transition (related to Bi-2201 phase) at about 23 K as a consequence of the existence of inhomogeneities in the oxidation state of superconducting grains. This fact is even verified by the degree of the broadening. Moreover, residual resistivity (temperature-independent) value tends to increase rapidly because of the impurity scattering and defect interfaces such as grain boundaries, stacking faults, voids, planar and micro defects, being favored by the bulk density and porosity analyses. Similarly, critical current density is measured to be in the range of 771 A/cm(2) (for the pure sample)-78 A/cm(2) (for the compound doped with x = 0.20). The critical current densities and temperatures are not measurable for the compounds substituted by the excess La concentration level (x>0.20) due to the beginning point of the metal to insulator transition (nonsuperconducting behavior). In other words, the disappearance of the superconductivity results from the destruction of the phase coherence (phase breaking). Additionally, surface morphology and interaction between the superconducting grains retrograde significantly with the La concentration. EDX measurement evidences also show that during the sample preparation procedure the elements each successfully enter into the Bi-2212 crystal structure and the observed peaks of Cu and especially Bi degrade regularly with the La inclusions, meaning that the foreign particles may substitute for the elements (especially Bi). Based on the XRD measurement results, a axis length is observed to elongate regularly due to both the induced electrons into anti-bonding orbital and ionic radius of La3+ > that of Bi3+ whereas the cell parameter c is obtained to shrink considerably as a result of the required extra energy for the bond formation in the orbital hybrid process. Moreover, the electrical conduction transition from the 2D variable range hopping (VRH) to phonon-assisted 3D VRH mechanism is theoretically discussed with the aid localization length, probable jump distance, average hopping energy and inelastic diffusion length parameters.
- 出版日期2015-2-15
