Flux‐pinning properties of nanocrystalline Hf O 2 added YB a 2 C u 3 O 7− δ superconductor

The effect of addition of different weight percentages of nanocrystalline HfO 2 , synthesized through a modified combustion method in bulk coarse‐grained orthorhombic YBa 2 Cu 3 O 7− δ powder was studied. The structure and microstructure of the samples examined by X‐ray diffraction (XRD) and scanning electron microscopy (SEM) showed the formation of a secondary phase YBa 2 HfO 5.5 in the superconductor without deteriorating the superconducting properties. The variation of sintering temperature, density, critical transition temperature ( T c ), magnetic field dependence of critical current density ( J c ), and flux‐pinning force ( F p ) of YBa 2 Cu 3 O 7− δ having different proportions of YBa 2 HfO 5.5 formed in the matrix were studied in detail. The samples showed a transition temperature ( T c ) of ∼92 K and the self‐field critical current density ( J c ) increases significantly up to an optimum value of 5.42 × 10 4  A cm −2 for 2 wt% addition of nano HfO 2 . The enhancement of the critical current density in the YBa 2 Cu 3 O 7− δ ‐HfO 2 samples is attributed to the presence of the nonreacting YBa 2 HfO 5.5 phase, formed due to the addition of HfO 2 as artificial pinning center. The formation of YBa 2 HfO 5.5 in YBa 2 Cu 3 O 7− δ bulk superconductor also enhances the pinning force density from 3 × 10 6 to 30.4 × 10 6  N m −3 at 77 K.