The effect of Ni2O3 doping on the properties of single domain GdBCO bulk superconductors fabricated by a new modified top-seeding infiltration and growth process

Single domain GdBCO bulk superconductors (20 mm in diameter) have been fabricated by top-seeding infiltration and growth process (TSIG) with a new solid phase of [(1-x)(Gd2O3+1.2BaCuO2) + x Ni2O3] (where x = 0, 0.02, 0.06, 0.10, 0.14, 0.18, 0.30, 0.50 wt%) , the effect of Ni2O3 additions on the growth morphology, microstructure, critical temperature Tc, magnetic levitation force and trapped flux of single domain GdBCO bulks have been investigated. The results show that single domain GdBCO bulks can be fabricated when x is in the range of 0~0.50 wt%; and the Gd211 particles are not affected by the Ni2O3 doping in the samples. The Tc of the samples decrease from 92.5 K (x=0 wt%) to 86.5 K(x=0.50 wt %) when the x increases from 0 to 0.50 wt %, which is caused by the substitution of Ni3+ on Cu2+ site; both of the levitation force and trapped filed of the samples increase first and then decrease with the increase of x, the largest levitation force of 34.2 N is obtained in the samples with x=0.14wt%, and the largest trapped field of 0.354 T is obtained in the samples with x=0.10 wt%. The change law of the levitation force and trapped field of the samples is closely relative to the doping content x. As we known, the doping of Ni2O3 can result in substitution of Ni3+ on Cu2+ site in GdBCO crystals, which can reduce the critical temperature Tc of samples; although Tc and the physical properties of the samples is reduced with the increase of the doping amount of Ni2O3, but at the same time, the substitutions of Ni3+/Cu2+ in GdBCO crystals can produce local lattice distortions, which can work as magnetic flux pinning centers to improve the properties of the samples. The highest Tc is obtained in the samples without any Ni2O3 additions (x=0), but the magnetic flux pinning force of samples is weak, so both of the levitation force and trapped field of the samples are lower relatively. When the doping content x ≤0.14 wt%, although the Tc is reduced but still with a value higher than 90 K, so the magnetic flux pinning force of the samples, from the lattice distortion defects increase caused by substitutions of Ni3+/Cu2+, increases with the increasing of doping content x, and result in an enhancement of levitation force and trapped field. When the doping content x is great than 0.14 wt%, the magnetic flux pinning force of the samples is still increasing with the increasing of x, but the Tc of the sample is significantly reduced and even lees than 90 K, and finally result in an decrease of levitation force and trapped field. Only when the doping amount of Ni2O3 is appropriate, both of the Tc and magnetic flux pinning force are of a relative optimal value, and lead to an enhancement of levitation force and trapped field.