Non-superconducting Particles Research Articles

Flux pinningin superconducting 123 materials

Mechanisms, features, and properties of strong flux pinning insuperconducting 123 materials are reviewed. These include pinningby nonsuperconducting particles of 211 or 422 phase, regions withlower critical temperature (Tc) such as a region where Basites are substituted for with light rare-earth elements and anoxygen-deficient region, and radiation-induced columnar defects. Inparticular, the pinning by lower-Tc regions which seem tobe associated with the well-known pronounced peak effect are focusedupon. It is speculated that these regions work as strong repulsivepinning centres resulting from the kinetic energy of the orderparameter under a significant proximity effect. This pinningmechanism is different from the usual attractive condensation energyinteraction which occurs in 211 or 422 particles. A discussion willbe given on the mechanism which brings about the peak effect.General guiding principles for the improvement of the flux pinningproperty will also be discussed.

Role of interface quality in vortex pinning by large nonsuperconducting particles

It is widely believed that large nonsuperconducting particles inhibit the flow of the electric current and result in lower critical current density due to the reduction of the cross sectional area. However, it will be shown that the spatial variation of the magnetic penetration depth can lead to pinning of the vortices and increased critical current density provided that the interface between the superconducting phase and the nonsuperconducting particles is sharp.

Evidence for pinning by (Sr,Ca) 2CuO y particles in partial-melting processed bulk Bi 2Sr 2CaCu 2O 8+ δ ceramics

Flux distributions of partial-melting processed Bi 2Sr 2CaCu 2O 8+ δ ceramics are obtained using magneto-optic imaging. In remanent states ( μ 0 H a=0 T), large amounts of trapped flux are observed along (Sr,Ca) 2CuO y particles embedded in the Bi 2Sr 2CaCu 2O 8+ δ matrix. Despite the relatively large size of these particles (up to 30 μm), the pinning effect is similar to that of Y 2BaCuO 5 particles in melt-processed YBa 2Cu 3O 7− δ . Furthermore, we discuss how the pinning capability of non-superconducting particles of different sizes and densities will show up in magneto-optic images.

Studies of high temperature superconducting thin films by transmission electron microscopy

High temperature superconducting thin films have been studied by transmission electron microscopy. Observations showed many superconducting and non-superconducting particles formed on the YBa 2Cu 3O 7 (YBCO) thin films synthesized by pulsed laser deposition, which increased the surface microwave resistance of the film. However, if the deposition conditions are selected properly, epitaxial single crystalline YBCO films can be obtained. The effect of resputtering deviates the deposited film from stoichiometric composition for the DC magnetron sputtering. An off-axis sputtering configuration can compensate this shortcoming. The low T c (∼98K) Tl 2Ba 2CaCu 2O 8 films showed many defects, but if the post annealing procedure is performed carefully, good quality Tl-2212 thin films can be obtained.

Defects in YBa 2Cu 3O 7−δ thin films studied by magneto-optics

By means of the magneto-optical Faraday effect, flux-density distributions are observed in laser-ablated YBa 2Cu 3O 7−δ thin films. The samples investigated are found to contain numerous defects of two different classes; (1) extended linear defects of weakly superconducting material and (2) small non-superconducting particles. The influence of these defects on the flux distributions is analyzed in detail using the concept of discontinuity lines of the currents [Th. Schuster et al. Phys. Rev. B 49 (1994) 3443]. The magneto-optical flux patterns clearly reveal that an application of a critical-state model to calculate the critical current density from magnetization data is impossible when a sample contains such defects. It is shown that even in the best samples the flux fronts are not homogeneous but consist of a large number of defect-induced parabolic flux plumes.

Growth mode of Cu-rich particles on laser-ablated YBa 2 Cu 3 O 7 thin films

The microstructure of the laser-ablated YBa2Cu3O7 (YBCO) thin film deposited on heated (100) SrTiO3 substrate was examined by transmission electron microscope. The particles on the film mainly consist of CuO and few CuYO2. Most of these non-superconducting particles nucleate on or near the surface of the film and protrude about 100-400 nm in height. A large amount of a-axis YBCO grains also exist in the film, which nucleate at the substrate surface and grow perpendicularly above the c-axis YBCO film. The YBCO thin film deposited under low oxygen pressure has very different microstructure compared with YBCO thin film deposited under high oxygen pressure.

Formation mechanism of high- Tc and critical current in (Bi,Pb) 2Sr 2Ca 2Cu 3O 10/Ag tape

X-ray diffraction method was used to characterise the phase composition and investigate the formation mechanism of the (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 phase from the precursor with (Bi,Pb) 2Sr 2CaCu 2O 8 as the main phase. The reaction is found to be a two-dimensional nucleation (random)-growth type, −[ ln(1 − F)] 1 2 = kt , where F is the conversional fraction of (Bi,Pb) 2Sr 2CaCu 2O 8 and t is the sintering time. The size of the critical current of the tape is quantitatively related to the conversional fraction of (Bi,Pb) 2Sr 2CaCu 2O 8 to (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 phase. At low fraction regime of (Bi,Pb) 2Sr 2CaCu 2O 8, the critical current of a tape shows no clear dependence on the remaining Bi-2212 phase, other factors such as grain alignment, colony size, contact between colonies, and fine nonsuperconducting particles become important in controlling the J c. Also, in this regime, the predominant weak links to be the colony boundaries rather than the Bi-2212 phase.

Composition and microstructural evolution of nonsuperconducting phases in silver-clad (Bi,Pb)2Sr2Ca2Cu3Ox composite conductors

The composition and microstructural evolution of nonsuperconducting phases during the course of formation of (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) in a silver sheath have been investigated by x-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), and digital image analysis. Wire samples fabricated by the oxide-powder-in-tube technique were heat-treated under a variety of conditions (time, temperature, and oxygen pressure). Backscattered images taken on polished but unetched transverse cross sections were subjected to computerized image processing, which allowed determination of the stoichiometry and quantification of microstructural characteristics (such as area fraction, size distribution, position, and orientation) of each nonsuperconducting particle. The dominant nonsuperconducting phases observed by SEM/EDX were CuO, (Ca,Sr)2CuO3 (2/1), and (Ca, Sr)14Cu24O41 (14/24) in amounts that varied depending on the annealing temperature, time, and oxygen partial pressure. Time evolution studies performed at 825 °C in 0.075 atm O2 showed that the area fraction of 2/1 decreased with reaction time, while that for 14/24 increased. In all cases, a substantial amount (>10% area fraction) of nonsuperconducting phases was detected even after all the Bi2Sr2CaCu2Oy (Bi-2212) in the as-rolled composite conductor was fully converted to Bi-2223, as determined by XRD. High aspect ratio nonsuperconducting particles were initially randomly oriented in the composite conductor core but gradually aligned parallel to the silver/(Bi,Pb)-Sr-Ca-Cu-O interface after extended annealing. They tended to segregate and exhibited a much broader size distribution when processing was carried out at temperatures and oxygen partial pressures on the high end of the normal processing range, most likely as a result of the occurrence of partial melting in the system.

Irreversibility line and flux pinning properties in high-temperature superconductors

The current-voltage characteristics are analysed numerically at high temperatures in terms of the one-dimensional flux creep model. A melt-processed Bi-2212 material including fine non-superconducting particles is considered as an example. A virtual critical current density in the creep-free case is assumed to approach the experimentally observed value at low temperatures, and a theoretical expression based on the flux pinning by non-superconducting particles at high temperatures where the effect of flux creep is significant. The critical current density is determined using the electric field criterion and the irreversibility field is obtained using a suitable criterion. The obtained irreversibility field shows different temperature dependences between low- and high-temperature regions as observed experimentally. The scaling characteristics of pinning force density are also discussed.

Effect of short coherence length along the c-axis on the irreversibility line in high- Tc superconductors

It has been empirically known that the irreversibility line in quasi-two-dimensional high- T c superconductors is farther from the phase boundary, B c2( T), than in three-dimensional ones. Such a difference was investigated in terms of the flux creep theory from the viewpoint of the coherence length along the c-axis, ξ⊥. For superconductors with short ξ⊥ in the field normal to the c-axis, the flux bundle size is small due to the small shear modulus C 66 of the fluxoid lattice, resulting in small pinning potential depth. Hence, even if strong pinning centers such as fine non-superconducting particles are successfully introduced into the materials, their effect on the irreversibility line is not as large as that in three-dimensional superconductors. However, this inherent inferiority can be overcome up to a sufficient level for practical applications by improvement of the flux pinning characteristics. The case is also treated where the magnetic field is applied parallel to the c-axis. In this case, the irreversibility line is restricted to a low level because of the small B c2 value.

Irreversibility line in superconducting Bi-2212 single grain with fine normal particles

The irreversibility line was obtained in terms of the imaginary a.c. susceptibility measurement for a melt-processed superconducting Bi-2212 single grain with fine non-superconducting particles. This line showed different temperature dependences in high and low temperature regions. The one in the low temperature region agrees approximately with the prediction from the flux creep theory with experimentally observed pinning parameters. On the other hand, the one in the high temperature region is close to the well known characteristics in Y-based superconductors, suggesting flux pinning by boundaries such as the surfaces of non-superconducting particles. A quantitative discussion is also given on flux bundle size.

Flux pinning by non-superconducting inclusions in melt-processed YBaCuO superconductorrs

While non-superconducting particles are known to serve as effective pinning centres in conventional superconductors, their effect in high T c superconductors is still controversial. In this paper, we give evidence that non-superconducting Y 2BaCuO 5 (211) inclusions can act as pinning centres in melt-processed YBaCuO superconductorrs even when their size is orders of magnitude larger than the coherence length.

Flux pinning due to nonsuperconducting particles in melt processed YBaCuO superconductors

While nonsuperconducting particles are known to serve as effective pinning centers in conventional superconductors, their effect in high T c superconductors is still controversial. In this paper, we give evidence that nonsuperconducting Y 2BaCuO 5 (211) inclusions can act as pinning centers in melt processed YBaCuO superconductors even when their size is orders of magnitude larger than the coherence length.

ＭＰＭＧ法で作製したＹＢａＣｕＯによる磁気浮上

Pinned type II superconductors can repel magnetic fields due to flux pinning. The recently developed Melt-Powder-Melt-Growth process enabled us to introduce fine non-superconducting particles into YBa2Cu3Ox matrix and to achieve large flux pinning force. It was found that an addition of silver was effective to reduce the amount of cracking and to increase the repulsive force of the superconductors against magnets. A heavy object could be levitated above the MPMG grown YBaCuO-Ag cooled with liquid nitrogen. We have also succeeded in levitating a person.