Correlation between superconducting properties and microstructure of and ceramics fabricated by the citrate gel process

Abstract

The correlation between microstructure and superconducting properties of (deficient (D) samples) and (substituted (S) samples) fabricated by the citrate gel process has been studied. In both types of samples nonsuperconducting phases such as and CuO were observed for . EDX analysis of S samples by TEM and STEM revealed the Ag content in YBCO grains to be lower than the nominal one (x) for all x in a range . Ag segregated mainly as small precipitates in boundaries, triple junctions and grains. The dependence of the critical current density on the magnetic field B obtained showed that is controlled by weak links in both cases, D and S. The measurements of at temperatures close to indicated that the weak links have SIS character in D and SNS character in S samples. Ag doping leads to an increase in the proportion of clean boundaries in S samples compared with D samples. Specific Ag precipitations of 2-5 nm size on or near clean boundaries were found in S samples for x = 0.4. This is most likely to be the reason for the improvement of in these samples by a factor of 3. Some increase in with x in D samples was observed and was probably associated with an increase in the proportion of faceted boundaries among clean ones.

Effect of ZrO2doping on structure and superconducting properties of sintered DyBaCuO ceramics

“…The grain size decreased strongly with the amount of doping (table 1). Precipitations of the secondary phases which may interfere with the grain growth during sintering are the most probable reason for the formation of a fine-grain microstructure, as earlier noted for Cu-deficient YBCO ceramics in [4,5].…”

“…Doping by different elements can substantially change the microstructure of GBs. Thus, in the Cu-deficient samples with a small deviation from stoichiometry, doping by Ag or Pt led to a substantial increase in portion of clean GBs in a sample [4,5]. Addition of different elements can also cause the formation of fine precipitations on or near clean GBs, which contribute highly to the value of J c , as, for example, was observed in [4] for samples doped by Ag.…”

“…We made a systematic statistical study of GBs similar to that performed earlier for YBaCuO ceramics doped with Ag [3,4] and DyBaCuO doped with Pt [5]. So 40 to 50 consecutive boundaries in each sample were characterized by bright field tilting imaging in TEM.…”

“…It is well known that in sintered bulk high-T c superconductors, the superconducting properties are mainly determined by their grain boundary (GB) behaviour [1,2]. The value of the critical current density J c depends primarily on the portion of clean boundaries in a sample, the crystallographic misorientation (relationship) between the adjacent grains and their chemical composition [3][4][5].…”

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

“…The character of the weak-link network was found in a first approximation from I c (T ) dependence as discussed in [17][18][19]. It turned out that Ca doping with z 0.1 leads to a change in the weak-link network character from superconductorinsulator-superconductor (SIS) to superconductor-normalmetal-superconductor (SNS) type (figure 4).…”