H. Küpfer

Investigation of inter- and intragrain critical currents in high Tc ceramic superconductors

Abstract Polycrystalline sintered bulk samples of REBa 2 Cu 3 O 7 with RE = Y , Gd were studied by a.c. susceptibility and an inductive measurement technique which allows the separation of intergrain (transport) from intragrain critical current density. Field and temperature dependence of the intergrain current are compared with theoretical predictions for a weakly coupled grain structure. Up to fields 0.3 T the weak link character of the intergrain current predominates. At higher fields it changes and points to a percolation current via superconducting grain boundaries. A quantitative correlation between intragrain current and twin spacing is hidden, probably by oxygen deficiency in the grain interior. No variation of the intragrain critical current density with temperature is observed up to 85 K in a textured specimen.

Intragrain junctions in YBa2Cu3O7−x ceramics and single crystals

Abstract Intragrain critical current densities of polycrystalline bulk samples and single crystals were investigated by inductive flux profile measurements. In addition to the well known granularity in a polycrystal, granularity within a single crystal was detected. In analogy to the polycrystalline samples increasing values of j c with decreasing size of the powder particles ground from single crystal gives strong evidence for the existence of intragrain junctions. The investigation of the field dependence of j c reveals as a common feature in ceramics and single crystals a maximum at fields far below the upper critical field. This observation and the behaviour of j c in the field region below this maximum are not in accordance with a critical current as expected from a pinning interaction, but rather reveal junction character. This is supported by data of fast neutron irradiation showing a lowering of the junction current, whereas above the critical current maximum j c increases by radiation induced pinning centres. The fields at which the maxima appear are in rough quantitative agreement with the irreversibility line from the glassy state or flux creep model. Once the phase coherence of the intragrain junctions is destroyed by the neutron damage, j c shows the expected continuous decrease with increasing field.

Fast neutron irradiation of YBa2Cu3O7

The influence of fast neutron irradiation on the superconducting transition temperature,Tc, the transition width, upper critical field,Bc2, and critical current density,jc, has been investigated in YBa2Cu3O7 up to a fluence of 1019 cm−2 (E>1 MeV). TheTc degradation with fluence is slightly less than in PbMo6S8, but larger than in A 15 compounds. The irradiation induced increase of the normal state resistivity is accompanied by a remarkable decrease of both the intergrainjc and the superconducting volume fraction.

Critical current and upper critical field of sintered and powdered superconducting YBa2Cu3O7

A sintered bulk sample of YBa2Cu3O7 has been investigated resistively and inductively with respect to its resistivity ϱ(B, T), upper critical fieldBc2(T), and current carrying capabilityjc(B, T). The inductive measurements were repeated after powdering the sample to a maximum particle size of 60 μm. Results show that the intrinsic-intragrain properties as approximately revealed by the powder measurements, are severely masked in bulk measurements by the intergrain weak-link structure. As an example, the current densityjc(≃0 T, 4.2 K) in the powdered YBa2Cu3O7 has a value of 4·106 A/cm2, which is about three orders of magnitude higher than in the bulk material. The high values of the intragrainjc are probably due to pinning at twin boundaries. — Besidesjc the normal state resistivity just aboveTc, ρn, is influenced by the interconnecting structure, both properties being directly related: a smaller ρn leads to a higherjc in bulk samples. — The upper critical field shows a pronounced anisotropyBc2|/Bc2⊥ of about 20 to 30, whereBc2|=330 T is the extrapolated upper critical field at zero temperature for the field orientation parallel to the (Cu−O)-planes. — For the time being, theBc2 anisotropy and the weak-link structure of sintered bulk samples are the main obstacles for high field applications.

Analysis of the dc-resistivity of YBa2Cu3Ox: Deduction of the intragrain resistivity

The resistivity ρ(100 K) scatters very much for YBa2Cu3Ox prepared as single crystals, epitaxial films or bulk samples which consist of grains of 1 to 10 μm diameter. An analysis of ρ(T) for granular bulk samples is presented indicating the existence of a low intrinsic resistivity ρi(T)=ρ0Li+αiT with ρ0Li≈0 and αi≃0.5µΩcm/K. The large grain boundary resistivity ρb between the grains (>1 μm) yield a macroscopic percolative conduction path lengtheningL/L0>1 with a reduced effective cross sectionC<C0 and thus, ρ(T)=Σρb+(LC0/L0C)·ρi(T). Evidence is presented for weak links inside single crystals and grains.

Weak link problem and intragrain current density in polycrystalline Bi1Ca1Sr1Cu2Ox and Tl2Ca2Ba2Cu3O10

Polycrystalline sintered specimen were investigated by means of ac susceptibility and inductive critical current measurements. Main objects are whether the new highTc oxides have a weakly coupled intergranular structure and to get information about the intragrain critical current density. The Tl2Ca2Ba2Cu3O10 specimen shows a connective nature similar to YBa2Cu3O7 with low intergrain currents (100 A/cm2 at 100K,B=0 T) and high intragrain ones (4.5·105 A/cm2 at 100K,B=0 T). The investigation of the Bi1Ca1Sr1Cu2Ox specimen becomes complex due to the presence of two superconducting phases and the low intragrain critical current density of the lowerTc phase.

Irreversible behaviour of oriented grained YBa2Cu3Ox. Part 1: transport and shielding currents

Abstract The critical current of oriented grained YBa 2 Cu 3 O x bulk material was investigated by resistive transport and inductive flux profile measurements. The applied magnetic field was directed parallel and perpendicular to the a–b plane. No features of granularity could be detected if the induced shielding current directed along the c-axis passed grain boundaries. The comparison of shielding and transport currents further ruled out the existence of intragrain junctions. The observation of a temperature scaling law for the volume pinning force allows comparison of the pinning strengths in both geometries. The volume pinning force is found to be about one order of magnitude larger if the magnetic field is parallel to the twin planes. Its field dependence resembles grain boundary pinning due to electron scattering, as observed in Nb 3 Sn. In the perpendicular geometry pinning may be explained by a collective interaction due to intrinsic or background pinning. The flux bundle volume, which may be identified with the correlation volume of the collective model, was experimentally determined.

Investigation of collective creep in YBaCuO

Abstract Time dependent decay of the magnetic moment M(t) and magnetization measurements were used to study flux creep in melt textured YBaCuO samples. From both measurements voltage current characteristics were reconstructed between 10 −1 and 10 −7 μV/cm up to the region of the irreversibility field. The exponent μ of the power law dependence of the pinning barrier was obtained from M(t) using the time constant of the initial stage of relaxation and from E(j) analysis. The current dependence of μ shows a maximum, negative values on both sides as well as the field dependence are not in agreement with theoretical predictions.

Flux creep and fishtail in melt-textured and single-crystalline YBa2Cu3O7

Time-dependent decay of the magnetic moment and magnetization measurements were used to study pinning and flux creep in two samples with and without aj(B) maximum in nonzero field (fishtail effect). From both measurements theE(j) relation was reconstructed forB∥c and the characteristic current exponentμ was obtained. At highj and lowB μ values are between 1 and 4; with increasingB μ passes a maximum and approaches negative values. At lowj μ is below 0.5 and not dependent onB orj. This behavior, which is qualitatively the same in both samples, is compared with proposed phase diagrams of the vortex lattice. Largeμ values are correlated with the plateau of the normalized creep rateS λ0.025; both observations indicate low relaxation and are found in that lowB region for whichj(B) has its minimum. This observation rejects a dominating influence of relaxation on the fishtail effect.

Critical Current Characterization of Melt-Processed YBa2Cu3O7−x

Upper critical field, irreversibility line, weak superconducting coupling and thermal relaxation are investigated in melt processed YBa2Cu3O7−x in the geometry magnetic field parallel and perpendicular to the a,b plane. Granularity within the highly c-axis oriented grains is excluded from comparison between transport and shielding current if the current flows within the a,b plane. The current along the c-axis is not degraded at the grain boundaries but becomes nonuniform with increasing field and temperature. This information was obtained from a more detailed investigation of magnetization measurements. Oxygen deficiency and inhomogeneous oxygen distribution are proposed as a major reason for this behaviour. Flux creep phenomena were studied by magnetic relaxation and inductive flux profile measurements. Temperature and field dependent activation energies were determined. The increase of the activation energy with temperature is discussed and compared with different explanations. Sweep rate dependent dc and ac measurements which allow access to relaxation in the millisecond regime exclude in the low temperature region a short time decay of the magnetic moment before the relaxation measurements starts.