Effect of Ca doping on microstructural and superconducting properties of DyBa2Cu3O7-δ/Caz (0 ≤ z ≤ 0.2) ceramics

Using bright-field and energy selective x-ray analysis techniques in transmission electron microscopy, we have studied the effect of Ca doping on the microstructural and superconducting properties in DyBa2Cu3Oy/Ca-z (z = 0, 0.05, 0.1 and 0.2) ceramics, i.e. for the case when Ca was added in excess to the stoichiometric 123 composition. It was found that in this case approximately half of the added Ca amount entered the 123-lattice. Thus, in the composition with nominal z = 0.1 (0.77 at%) average Ca concentration in the grains was about 0.4 at%. On the basis of measured atomic concentrations and 123 stoichiometry of grains, it was concluded that Ca predominantly substituted Dy in the bulk for all levels of doping (0 less than or equal to z less than or equal to 0.2), although for z = 0.2 the rate of substitution for Ba also became pronounced. This substitution of Dy3+ by Ca2+ seems to explain the observed decrease in T-c from 92 K in an undoped sample to 74-77 K for compositions with z = 0.1-0.2. Ca segregated in grain boundaries (GBs). The concentration of Ca in most 'clean' (secondary-phase-free) GBs was 1.5-3.5 times higher than in the adjacent grains. Ca segregated in a narrow region (d less than or equal to 5 nm) around the GB. In the majority of GBs, Ca substitution for Dy was favoured, although with increasing the doping level from z = 0.1 to z = 0.2 the rate of the substitution for Ba increased. This substitution for Dy most probably explains the obtained dramatic increase of J(c) at T less than or equal to 0.8 T-c in the Ca-doped samples. Ca segregation in GBs is the most probable reason for the superconductor-normal-metal-superconductor (SNS) behaviour of the GB network in Ca-doped samples. Ca doping led to a higher proportion of clean GBs, in the sample with z = 0.1, that could be responsible for a slower decrease of J(c) with magnetic field in this sample.