Effect of Ca doping on microstructural and superconducting properties of DyBa₂Cu₃O₇  /Ca_z(0  z  0.2) ceramics

Abstract

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 DyBa 2 Cu 3 O y /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 z 0.2), although for z = 0.2 the rate of substitution for Ba also became pronounced. This substitution of Dy 3+ by Ca 2+ 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 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 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.

Pinning centres and enhancement of critical current density in YBCO doped with Pr, Ca and Ni

“…This corroborates the observations in figure 4(c). This way it markedly helps to transform grain boundary junctions from S-I-S to S-N-S type as extensively reported elsewhere [8][9][10]. For a 2% doped sample the observed increase in j c , however, outweighs the increase in volume fraction which suggests the possible addition of new pinning centres which were previously not present in the pristine sample.…”

“…Incidentally, Zn produces the largest T c depression [7] and is found to have a relatively larger effect on j c (B) than Ni [5]. Besides the above dopants, several authors [8][9][10] have reported Ca substitution to dramatically improve intergranular j c of RE-123 by transforming the grain boundary S-I-S junctions to S-N-S type to marginalize the weak link effect. Ca, at grain boundaries, is believed to significantly lower the boundary resistivity.…”

The effect of oxygenation temperature on the distribution of Ca in Dy123/0.2Ca ceramics and superconducting properties

“…A complete list of the studied samples is given in table 1. It is seen that Ca doping leads to a substantial decrease in grain size, as was already noted in our previous paper [6]. The normal state resistivity at room temperature was nearly the same in undoped and Ca-doped samples for both T oxg .…”

“…The obtained difference between the nominal and real concentration of Ca in 123 grains can be associated with the presence Ca in secondary phases. As already shown in [6], Ca promotes the formation of some new (Dy)(Ba)(Cu)O phase in the proportion 1:(1.2-1.5):(2-4):(4-8) with 0.5-2.0 at.% Ca and in the proportion 1.2:1:(2-2.5):(3.5-4) with 0.15-0.5 at.% Ca. Calcium also entered in a very limited amount ( 0.15 at.%) into the 'green' phase.…”

“…In ceramics, grain boundary segregation effects can greatly contribute to changes in intergranular J c . We have shown that in the fully oxygenated 123/Ca z (0 z 0.2) ceramics, most GBs exhibited strong Ca segregation [6]. In doing so, Ca substitution occurred practically only on Dy sites for z 0.1, but for z = 0.2, although substitution of Dy by Ca was predominant, Ca substitution for Ba was also noticeable.…”