Microstructure and superconducting properties of sintered DyBaCuO ceramics doped by Ca

Abstract

The effect of Ca-doping on the microstructure and superconducting properties of sintered stoichiometric Dy1−xCaxBa2Cu3O7−δ (S samples) and Cu-deficient Dy1−xCaxBa2Cu2.8O7−δ (D samples) (x ≤ 0.3) ceramics was studied. The systematic characterization of the fine microstructure was done via conventional and high-resolution observation in TEM as well as fine local energy dispersive x-ray (EDX) analysis in TEM. The distributions of the different types of grain boundaries (clean or coated with a thin or thick film) were statistically studied. A correlation between Tc and the microstructure of grains was found in the studied ceramics. Doping by Ca resulted in a change in Tc from 92 K to 81 K in the S sample (x = 0.2) and from 92 to about 61 K in the D samples (x = 0.2, 0.3). The obtained difference in Tc in the S and D samples is likely to be associated with a different type of Ca-substitution. In D samples Ca mostly substituted Dy, whereas in the S sample the replacing of Dy and Ba by Ca was approximately equiprobable. Doping by Ca (x ≤ 0.3) substantially increased Jc at T ≤ 50 K and led to some stabilization of Jc in low magnetic fields (B ≤ 80 G) in the Cu-deficient samples. This seems to be explained by the increase in the proportion of clean GBs and the improvement of their intrinsic transport properties due to Ca-doping of these compositions. The measured dependence of the critical current Ic on temperature close to Tc indicated that the grain boundary network in D samples changed the weak-link character from SIS to SNS type as a result of Ca-doping. This change may be explained by the substitution of Dy by Ca at GBs in these compositions. Enhanced concentration (segregation) of Ca was detected in ‘clean’ GB with big misorientation θ ≥ 20° in all the studied compositions doped by calcium.

Intergranular flux pinning in underdoped and overdoped R1−xCaxBa2Cu3Oz (R=Y, Gd; x=0, 0.2) samples

“…The observed narrowing of intergranular transition could be a result of improvement of intergrain connections and intergranular resistance reduction [23]. All this is in consent with Ca segregation in GB regions, reported by different authors [8,24] Second, the intergranular transition shift, with increasing the frequency, is smaller in Ca substituted sample than in non substituted one. Higher level of overdoping leads to a higher intergranular current, supporting intergranular flux pinning in Ca substituted sample.…”

“…Using this analogy we can suppose that normal carriers and/or small regions of normal 10 state are present at grain boundaries of overdoped samples. It is known that Ca segregates at the grain boundary regions in substituted Y123 phase [8,24,33].…”

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

“…However, the incorporation of Ca on Dy sites in grains led to a substantial decrease in T c of the sample. In the case of predominant Ca substitution for Ba, the intergranular critical current was not improved, but the decrease in T c was much less than for the substitution of Dy [12]. Substitution of Y by Ca in YBa 2−y Ca y Cu 3 O 7−δ was also studied in [13,14] where it was found that, even for a high level of doping (y = 1), T c decreased by about 10 • only.…”

“…In [12] we studied the influence of Ca doping on superconducting properties and, first of all, on J c for bulk DyBCO ceramics in which Ca was added as substituent for Dy in amounts equal to Dy deficiency (Dy 1−z Ca z Ba 2 Cu 3 O 7−δ , 0 z 0.3). Using energy dispersive x-ray (EDX) analysis in transmission electron microscopy, it was found that for a large concentration of Ca (z = 0.2-0.3) Ca replaced not only Dy, but also Ba partly.…”

Scaling Behavior of Current- Voltage Characteristics of Y1-xCaxBa2Cu3O7-δPolycrystalline Samples

“…The substituted atoms produced nanoregions in the material with suppressed T c, and may serve as effective intragranular pinning centers [16,17]. In particular, it has been shown that a small quantity of Ca (2-4%) increases the pinning in Y 1-x Ca x Ba 2 Cu 3 O 7-samples [17][18][19]. The accompanying effect of oxygen vacancy creation, especially in CuO 2 planes should be also important for pinning.…”