The composite ceramics of (YBa2Cu3O7−δ)1−x(Fe3O4)x

The effect of sintering temperatures on the superconducting properties of the composites of (YBCO)0.975(Fe3O4)0.025 (2.5 wt.%) has been studied. The field-cooled measurement finds that with the sintering temperature increasing the superconducting fraction is increased first and then decreased. The optimum sintering temperature is found. The Raman vibration modes (out-of-phase bond-buckling mode and in-plane bond-stretching mode) are recorded at room temperature. The extended X-ray absorption fine structure proves that no further lattice distortion occurs on YBCO after the addition of Fe3O4, but the local structure disorder of YBCO has altered as the sintering temperatures changed. Compared the shift of the Raman phonon modes with the magnitude of oscillations of extended X-ray absorption fine structure in R space it can be concluded that the local disorder has relation with superconducting properties. Scanning electron microscopy images show that the doped Fe3O4 are diffused into the inter-grain sites. With sintering temperature increasing the small-size grains get fused to form larger grains. The strain produced by grain interaction leads to the change of local structure disorder. Whereas, further increase of the sintering temperatures, some Fe ions at the grain boundaries are incorporated into the unit cell of YBCO, which leads to the decrease of superconducting fractions. By X-ray absorption near edge structure, it is found that the white lines occurring near K edge of Cu2+ show an energy shift toward high energy side. It can be considered that as the sintering temperature increases more holes are formed and the screening effect around the Cu2+ weakens. The screening behavior is also conducive to the formation of superconducting fractions.