Behavior Of Critical Current Density Research Articles

Angular behaviour of critical current density in YBa2Cu3Oy thin films with crossed columnar defects

Columnar defects (CDs) were installed into YBa2Cu3Oy thin films at the crossing angle ±θi relative to the c-axis using 200 MeV Xe ions, in order to clarify the influence of ±θi on the angular behaviours of critical current density Jc. The width of a Jc peak occuring at B || c linearly increases with broadening CD-crossing angle, where its shape maintains a single peak in the range of θi ≤ ±25°. The CDs crossing at ±45°, on the other hand, drastically change the angular behaviour of Jc around B || c: the Jc is equally elevated over the angular region from −45° to 45° in the lower magnetic field, whereas the rapid reduction of the Jc occurs at B || c with increasing magnetic field, leading to the appearance of double peaks near the irradiation angles, ±45°. This crossover phenomenon is attributed to the difference in the pinned states of flux lines between B || c and at the irradiation angles: the CDs crossing at ±45° cannot provide the correlated flux pinning along the c-axis, resulting in the uncorrelated pinned state at B || c.

Significant tunability of thin film functionalities enabled by manipulating magnetic and structural nano-domains

The influence of laser frequency on the structure and physical properties of thin films grown by pulsed laser deposition has been studied. Different types of thin films, hard ferromagnetic FePt L10 and quasi-single crystal superconducting YBa2Cu3O7 (YBCO), have been used for demonstration of the effect. Significant structural modifications have been obtained for the films with similar thicknesses. These modifications are shown to dramatically control their corresponding properties, providing an instrumental ability for tuning the practical characteristics of the films by changing the laser frequency of their deposition. In particular, 20-fold increase of coercive field and modification of demagnetization mechanism are obtained for FePt films by varying the frequency from 1 Hz to 6 Hz. Over a similar frequency range, a strong dependence on the laser frequency is discovered for the YBCO films for the critical current density behavior as a function of the applied magnetic field [Jc(Ba)] with the unexpected reversal of Jc(Ba) curves with temperature. The mechanisms of structure modifications and corresponding property variations are proposed.

Improved Performances of $\hbox{MgB}_{2}$ Conductor by Using of Innovative Amorphous and Nano-Structurated Boron

In this paper, we report about improved performances of amorphous and nano-structurated boron in MgB2 critical current density of wires and tapes. The boron produced by magnesiothermic reaction, using nano-structurated boron oxide, is used to synthesize nano-structurated MgB2 powders. These powders are used to produce several wires by ex-situ P.I.T. method. Here we applied only the ex-situ technique, in which our group is very well skilled. The same procedure can be applied to obtain in-situ P.I.T. tapes and wires as well. The boron and the MgB2 powders are characterized both from a physical-chemical point of view and from the superconductive point of view. We performed X-ray analysis to compare the home-made boron crystalline degree with respect to the commercial one. SQUID and transport measurements were done to compare the wire's critical current density behavior with respect to our past results using different boron.

Description of the transport critical current density behavior of polycrystalline superconductors as a function of the applied magnetic field

Abstract A model to describe the critical current density behavior of high- T c polycrystalline superconductors is proposed for all magnetic field values. The main features of the model are as follows: the transport critical current density is controlled by the weak-link network at grain boundaries. The size distribution of weak links is well represented by a Gamma-type distribution. Finally, the tunneling critical current between grains follows a Fraunhofer diffraction pattern or a modified pattern if the applied magnetic field is lower or higher than the first critical field H c 1 .

Influences of Microstructure on Critical Current Properties in MgB2 Superconducting Bulk Fabricated using a Premix-PICT Method

We report relationships between the superconducting properties, microstructure and fabrication conditions in high-density MgB2 bulks fabricated using a premix-powder-in-closed-tube (PICT) method. The microstructure of the MgB2 bulks depends on the crystallinity of the initial B powders; use of crystalline B powders reduces the amount of MgB2 and increases re-sidual B and MgO. The heat treatment condition influences the grain sizes of MgB2 as well as the distribution and grain sizes of MgO. A lower temperature and a shorter duration of heat treatment tend to form fine grains of MgB2 and uniform dispersion of fine MgO particles. These microstructural variations influence the distribution of effective flux pinning centers such as grain boundaries and fine MgO particles. Different types of critical current density behavior as a function of magnetic field were meas-ured.

Effects of ambient and humidity exposure on the MgB 2 superconductor

Exposure effect of ambient (up to a period of 465 days) and humid atmosphere on stability and the physical properties of bulk samples of MgB 2 has been carried out based on ac-susceptibility and X-ray diffraction measurements performed at different times during the aging. The motivation of this study came from the device application potential of MgB 2 superconductor. It has been observed that on initial exposure to ambient for a period of about 2 months, the MgB 2 samples neither show any sign of phase decomposition nor any significant degradation of superconducting parameters. However, on further exposure to ambient for a period greater than 4 months duration, the samples progressively exhibited the partial conversion of MgB 2 phase into Mg-deficient phases, viz., MgB 6 and MgB 12 and also leading to formation of Mg(OH) 2 which is accompanied by decrease in the T C and poor connectivity of the superconducting grains. When exposed to humid environment for a period of 30 days, a significant degradation was observed in the superconducting properties of MgB 2. In sharp contrast to the case of ambient exposure, the exposure of MgB 2 to humid atmosphere (for 30 days) did not result in any noticeable phase decomposition. A comparison of the magnetically inferred critical current density behavior before and after the humidity exposure of MgB 2 is also presented.

Enhancement of critical current density of YBa 2Cu 3O 7 − δ thin films by self-assembly of Y 2O 3 nanoparticulates

YBa 2Cu 3O 7 − δ (YBCO) thin films, possessing high critical current density ( J c), have been synthesized by embedding a homogeneous array of Y 2O 3 non-superconducting nanoclusters/nanoparticles using a pulsed laser deposition technique. The size, interparticle spacing, and density of Y 2O 3 nanoparticles in YBCO thin films were tailored by varying the number of laser pulses in order to determine the optimum size for effective immobilization of vortices. Scanning transmission electron microscopy with atomic number contrast and X-ray diffraction techniques were used to determine the size and structure of the nanoparticles. Both techniques indicate that the Y 2O 3 particles are epitaxial with respect to the surrounding YBCO matrix. The information about pinning of vortices by the nanoparticles was obtained by investigating the behavior of critical current density as a function of temperature and applied field, which in turn determines the vortex density in the sample. The superconducting transition temperature ( T c) of YBCO films with the inclusion of nanoparticles was observed to remain almost the same or decrease marginally (1–2 K) with respect to T c of pure YBCO films deposited under identical conditions. However, J cs of YBCO films embedded with self-assembled nanoparticles were found to be significantly higher than that of pure YBCO films. The J c enhancement was up to five times in high magnetic field, which is a key requirement for practical application of high- T c materials.

The effect of pre-sintering and deformation rate on critical current density behaviour of Bi-2223 bulk samples made by sinter forging

A technique combining cold-isostatic-pressing (CIP) and sinter forging was applied to the fabrication of Bi-2223 bulks, and pre-sintering was conducted after (route 1) and before CIP (route 2). Both of them involve sinter forging twice and no post-annealing. The effect of pre-sintering and deformation rate on the critical current density Jc is investigated. Such obtained Bi-2223 bulks had a cross-sectional area of ∼1 cm2 and therefore the total critical current is close to meeting some technical applications. The Jc results from route 1 are much higher than from route 2 although both reached highest Jc at the deformation rate 80%. The main reason is that route 1 produces much larger, plate-like, better-contacted grains whereas by route 2 much smaller, round-shaped and poorly contacted grains were obtained.

Phenomenological model for history effects and metastability in weakly pinned superconductors

We present a phenomenological model to describe features in the hysteretic magnetic response of weak pinning superconductors across the peak effect region. It accounts for the experimentally observed history dependent behavior of critical current density and the metastability of the vortex state prior to and across the peak effect region of superconducting systems such as ${\mathrm{NbSe}}_{2},$ ${\mathrm{CeRu}}_{2},$ and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}.$ Moreover, this model reduces to Bean's critical state model as a limiting case.

Behavior of Critical Current Density and Grain Misalignment of Hot-Pressed Bi-2223 Bar Current Leads

Bi-2223 bar current leads were fabricated by a combination of cold isostatic pressing (CIP) and hot pressing (HP). The critical currents were measured at 77 K by varying external dc magnetic field (B), which was applied parallel and perpendicular to the bulk sample surface, i.e., Ic (B//bar surface) and Ic (B ⊥ bar surface), respectively. The critical current Ic and critical current density Jc were 119 A and 300 A/cm2, respectively, for the bar current lead of 0.4 cm thick, 1.0 cm wide, and 5.4 cm long. The effective grain misalignment angle, ϕeff, was around 10°, calculated from its Ic (B//bar surface) and Ic (B ⊥ bar surface) curves by using 2D model. For comparison, the average grain misalignment angle, ϕav, found in microstructure using SEM, was measured by pole figure. The obtained results were in agreement with the effective grain misalignment angle ϕeff. The phases and microstructure were analyzed by XRD and SEM.

Status of Bi-2223 tapes performance and development

We are developing Bi-2223 superconducting tapes on a wide viewpoint from fundamental superconducting property to application usage. In basic characterization of superconductivity, the behavior of critical current density (J/sub c/) in terms of both temperature and magnetic field was studied in detail and a new scaling law which can estimate the value of J/sub c/ in multiple conditions is defined. There were significant improvements in the long length fabricating process. The 1 km order length tape having J/sub c/ at 77 K over 20 kA/cm/sup 2/ can be produced reproducibly. The distribution of J/sub c/ through the whole length is very uniform. These results in both basic characterization and production progress made it possible and easy to design a high field magnet. Then, we have successfully developed a magnet using a refrigerator.

Anisotropy of fishtail effects in EuBa2Cu3Oy crystal prepared by melt-textured growth method

The behavior of critical current density in EuBa2Cu3Oy crystal prepared by melt-textured growth method were studied. The fishtail effects were observed for H//C and H⊥C. For H//C the fishtail effects were explained by collective pinning theory, whereas for H⊥C it has been attributed to matching effects.

Scaling behavior of fishtail effects in YBa2Cu3Oy crystal prepared by melt-textured growth method

The scaling behavior of critical current density in fishtail effects has been observed in YBa2Cu3Oy crystal prepared by melt-textured growth method, and has been ascribed to the phonon-like fluctuation of vortex flux lattice.

Facet Topography and Dislocation Structure as a Possible Source for Electrical Heterogeneity in [001] TILT Bicrystals of YBa2Cu3O7-δ

A current theme in the study of the critical current density behavior of YBa2Cu3O7-δ (YBCO) grain boundaries is that their electromagnetic properties are heterogeneous on various length scales ranging from 10s of microns to ˜ 1 Å. Recently, combined electromagnetic and TEM studies on four flux-grown bicrystals have demonstrated a direct correlation between the length scale of the boundaries’ saw-tooth facet configurations and the apparent length scale of the electrical heterogeneity. In that work, enhanced critical current densities are observed at applied fields where the facet period is commensurate with the spacing of the Abrikosov flux vortices which must be pinned if higher critical current density values are recorded. To understand the microstructural origin of the flux pinning, the grain boundary topography and grain boundary dislocation (GBD) network structure of [001] tilt YBCO bicrystals were studied by TEM and HRTEM.

Overcoming weak links at grain boundaries in Bi2Sr2CaCu2Ox thick films by incorporation of superconducting whiskers

Bi2Sr2CaCu2Ox (Bi-2212) superconducting thick films incorporating Bi-2212 superconducting whiskers have been prepared using a partial melting method. Hysteresis behavior of critical current density (Jc) for magnetic field is observed in a film without whiskers (monolithic film), but not noticeably in composite film (whisker content: 20 wt %). It is found from ac susceptibility measurement that a peak in χ″, resulting from weak links, appears at a higher temperature for the composite film than for the monolithic film. Jc decreases with increasing magnetic field more rapidly for the monolithic film than for the composite film less than 1 T. These results indicate that the incorporation of the whiskers is effective for overcoming weak links. In consequence Jc for the composite film is about three times higher than for the monolithic film. It is clear from SEM observation that a microstructural improvement leads to an increase in Jc.

Characteristics of TlBa2Ca2Cu3O9+δ powder as-synthesized and after grinding

Crystal structure and superconductivity of the Tl–Ba–Ca–Cu–O powder prepared by a solid-state reaction were studied. The results of x-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses indicate that the powder of a major single 1223 phase was synthesized with a cauliflower-like morphology; by grinding, the powder was partially transformed to an amorphous phase. The measurement of magnetic hysteresis was also performed in the temperature range of 7–75 K up to 5 T from which the weak field-dependent behavior of critical current density was observed. The as-synthesized powder, with Tc = 110 K measured from magnetometer and susceptometer, is considered to consist of weak-linked regions. The results obtained from comparing the as-synthesized and the ground powder imply that in the Tl-1223 system, the Ag-sheathed tapes fabricated by the powder-in-tube process may be benefited by forming other intermediate phases with plate-like morphologies to give better densification or grain alignment.

Electric, magnetic, and transport critical current density behavior of the 2201, 2212, and 2223 phases of ?BSCCO?

We have studied the electrical properties of polycrystalline pellets of the high-T c superconducting phases occurring in the Bi2O3-SrO-CaO-CuO (BSCCO) system, having nominal compositions Bi2Sr2Cu1O6+y, (2201 phase, withT c = 10 K.) Bi2Sr2CaCu2O8+y (2212 phase, withT c = 85 K), and Bi2Sr2Ca2Cu3O10+y (2223 phase, withT c = 110 K). Pellets containing the 2223 phase having zero resistance below 105 K were obtained. For the 2212 and 2223 phases, the transport critical current density was measured as a function of temperature and of the externally applied magnetic field. As previously found for YBa2Cu3O7-x (YBCO), results are consistent with the occurrence of a weak superconducting coupling among the grains. Such coupling was, however, noticeably stronger for the 2223 phase than for the 2212 phase.