ure 6. As it can be seen from this figure, the resistivity of YBa2(Cu1−xCox)3O7−δ samples vs. temperature shows that for the onset transition temperature is Tconset ~ 88˚K, 82˚K, 81˚K and 80˚K for x = 0, 0.002, 0.004 and 0.006 Co respectively. Decrease in the number of hole carriers by impurity doping to the CuO chain should be responsible for reduced Tc of YBCO phase Cobalt doping, this reduction has been estimated to 2 - 8 K/at% for trivalent dopants (M: Fe, Co and Al) [23] . All samples show metallic behavior in the normal state, with high resistivity comparing to undoped sample. The residual resistivity (ρ0) is found by linear extrapolation of the resistivity curve in the range from 2Tc up to room temperature.

Figure 6. Variation of the resistivity as function of Co content in YBa2(Cu1−xCox)3O7−δ.

Figure 7. The onset of superconducting transition temperature Tco and residual resistivity variation ρ0 as function of Co content in YBa2(Cu1−xCox)3O7−δ.

As an inherent property of conducting materials, ρ0 is modified progressively with the added composites (Figure 7). This signifies that inter granular connectivity is also modified by these composites. The residual resistivity is considered as an indicator of the sample homogeneity and defects density. The higher residual resistivity value corresponding to the sample doped with x = 0.06, these results are confirmed by the porous structure obtained in the micrographs analysis. Co doping makes the temperature dependence of the resistivity ρ (T) deviate from linear behavior at T, especially for the sample doped with x = 0.04 and x = 0.06. This deviation is related to the opening of a pseudogap. As discussed for oxygen depleting crystals [24] .

4. Conclusion

The effect of Cobalt doping on the microstructure and the normal state transport properties of polycrystalline YBCO was studied. Samples have been elaborated by the solid state reaction method and characterized by means of XRD, SEM, DTA and resistivity measurements. The crystal lattice parameters are found to change due to the cobalt doping and tendency to a structure phase transition from orthorhombic to tetragonal, which is confirmed by the decrease of the degree of orthorhombicity (b − a)/(b + a). The variation of full width half maximum (FWHM) of (00l) peaks as function of Co content reveals that there is a microstructural disorder caused by the cobalt incorporation. The thermal analysis of our samples reveals that the YBCO decomposition temperature is greatly lowered. In addition, the morphology examination with SEM revealed gradual increases of grain size with x = 0.02 Co; a high porosity is observed in the doped samples compared to the undoped. The resistivity measurements show a deviation from linearity of ρ(T), it is due to the opening of a pseudogap in Co doped samples.

Cite this paper

Chamekh, S. and Bouabellou, A. (2018) The Effects of Magnetic Dopant on the Structural and Electrical Properties in Superconducting YBaCu3O7−δ Ceramic. Advances in Chemical Engineering and Science, 8, 1-10. https://doi.org/10.4236/aces.2018.81001


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