Microstructure and superconducting properties of sintered DyBaCuO ceramics doped by Ca

• Published in: Superconductor science & technology 2002-08, Vol.15 (8), p.1244-1251, Article 314
• Main Authors: Laval, J Y, Orlova, T S
• Format: Article
• Language: English
• ISSN: 0953-2048; 1361-6668
• DOI: 10.1088/0953-2048/15/8/314

The effect of Ca-doping on the microstructure and superconducting properties of sintered stoichiometric Dy sub(1-x)Ca sub(x)Ba sub(2)Cu sub(3)O sub(7- delta ) (S samples) and Cu-deficient Dy sub(1-x)Ca sub(x)Ba sub(2)Cu sub(2.8) O sub(7- delta ) (D samples) (x less than or equal to 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 T sub(c) and the microstructure of grains was found in the studied ceramics. Doping by Ca resulted in a change in T sub(c) 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 T sub(c) 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 less than or equal to 0.3) substantially increased J sub(c) at T less than or equal to 50 K and led to some stabilization of J sub(c) in low magnetic fields (B less than or equal to 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 I sub(c) on temperature close to T sub(c) 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 theta greater than or equal to 20 degree in all the studied compositions doped by calcium.