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See detailThe influence of a grain boundary on the thermal transport properties of bulk, melt-processed Y-Ba-Cu-O
Marchal, C.; Fagnard, Jean-François ULg; Shi, Y. H. et al

in Superconductor Science and Technology (2013), 26

We report the dependence of thermal conductivity, thermoelectric power and electrical resistivity on temperature for a bulk, large grain melt-processed Y-Ba-Cu-O (YBCO) high temperature superconductor ... [more ▼]

We report the dependence of thermal conductivity, thermoelectric power and electrical resistivity on temperature for a bulk, large grain melt-processed Y-Ba-Cu-O (YBCO) high temperature superconductor (HTS) containing two grains separated by a well-defined grain boundary. Transport measurements at temperatures between 10 and 300 K were carried out both within one single grain (intra-granular properties) and across the grain boundary (inter-granular properties). The influence of an applied external magnetic field of up to 8 T on the measured sample properties was also investigated. The presence of the grain boundary is found to affect strongly the electrical resistivity of the melt-processed bulk sample, but has almost no effect on its thermoelectric power and thermal conductivity, within experimental error. The results of this study provide direct evidence that the heat flow in multi-granular melt-processed YBCO bulk samples should be virtually unaffected by the presence of grain boundaries in the material. © 2013 IOP Publishing Ltd. [less ▲]

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See detailLocalization of superconductivity in superconductor–electromagnet hybrids
Ataklti, G.W.; Aladyshkin, A.Yu.; Gillijns, W. et al

in Superconductor Science and Technology (2012), 25

We investigate the nucleation of superconductivity in a superconducting Al strip under the influence of the magnetic field generated by a current-carrying Nb wire, perpendicularly oriented and located ... [more ▼]

We investigate the nucleation of superconductivity in a superconducting Al strip under the influence of the magnetic field generated by a current-carrying Nb wire, perpendicularly oriented and located underneath the strip. The inhomogeneous magnetic field, induced by the Nb wire, produces a spatial modulation of the critical temperature Tc, leading to a controllable localization of the superconducting order parameter (OP) wavefunction. We demonstrate that close to the phase boundary Tc(Bext) the localized OP solution can be displaced reversibly by either applying an external perpendicular magnetic field Bext or by changing the amplitude of the inhomogeneous field. [less ▲]

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See detailMagnetic shielding properties of a superconducting hollow cylinder containing slits: Modelling and experiment
Fagnard, Jean-François ULg; Elschner, S.; Hobl, A. et al

in Superconductor Science and Technology (2012), 25(10), 104006

This paper deals with the magnetic properties of bulk high temperature superconducting cylinders used as magnetic shields. We investigate, both numerically and experimentally, the magnetic properties of a ... [more ▼]

This paper deals with the magnetic properties of bulk high temperature superconducting cylinders used as magnetic shields. We investigate, both numerically and experimentally, the magnetic properties of a hollow cylinder with two axial slits which cut the cylinder in equal halves. Finite element method modelling has been used with a three-dimensional geometry to help us in understanding how the superconducting currents flow in such a cut cylinder and therefore how the magnetic shielding properties are affected, depending on the magnetic field orientation. Modelling results show that the slits block the shielding current flow and act as an entrance channel for the magnetic flux lines. The contribution of the slits to the total flux density that enters the cylinder is studied through the angle formed between the applied field and the internal field. The modelled data agree nicely with magnetic shielding properties measured on a bulk Bi-2212 hollow cylinder at 77K. The results demonstrate that the magnetic flux penetration in such a geometry can be modelled successfully using only two parameters of the superconductor (constant J c and n value), which were determined from magnetic measurements on the plain cylinder. © 2012 IOP Publishing Ltd. [less ▲]

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See detailMagnetic properties of drilled bulk high-temperature superconductors filled with a ferromagnetic powder
Lousberg, Grégory; Fagnard, Jean-François ULg; Chaud, Xavier et al

in Superconductor Science and Technology (2011), 24

It is shown that filling the holes of a drilled bulk high-temperature superconductor (HTS) with a soft ferromagnetic powder enhances its trapping properties. The magnetic properties of the trapped field ... [more ▼]

It is shown that filling the holes of a drilled bulk high-temperature superconductor (HTS) with a soft ferromagnetic powder enhances its trapping properties. The magnetic properties of the trapped field magnet are characterized by Hall probe mapping and magnetization measurements. This analysis is completed by a numerical model based on a 3D finite-element method where the conductivity of the superconducting material is described by a power law while the permeability of the ferromagnetic material is fixed to a given value and is considered uniform. Numerical results support the experimental observations. In particular, they confirm the increase of trapped flux that is observed with Hall probe mapping after impregnation. [less ▲]

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See detailLocal probing of the vortex-antivortex dynamics in superconductor/ferromagnet hybrid structures
Silhanek, Alejandro ULg; Gladilin, V. N.; Van de Vondel, J. et al

in Superconductor Science and Technology (2011), 24(2),

In-plane ferromagnetic bars, densely packed in a linear array underneath a superconducting bridge, create two types of vortex chains of opposite polarity inside the superconductor. In this work we ... [more ▼]

In-plane ferromagnetic bars, densely packed in a linear array underneath a superconducting bridge, create two types of vortex chains of opposite polarity inside the superconductor. In this work we investigate both experimentally and theoretically the dynamics of these vortex chains as a function of an external magnetic field for two different arrangements of magnetic moments, namely parallel and antiparallel. The theoretical approach, based on the time-dependent Ginzburg-Landau formalism, confirms previously proposed empirical models implemented to describe the basic properties of these hybrid systems. In addition, local transport measurements allow us to probe the dynamics of individual vortex channels as a function of the applied magnetic field. These measurements evidence a drastic reduction of the dissipation in the channel populated with vortices having opposite polarity to the applied field. [less ▲]

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See detailSelf-heating of bulk high temperature superconductors of finite height subjected to a large alternating magnetic field
Laurent, Philippe; Fagnard, Jean-François ULg; Hari Babu, N. et al

in Superconductor Science and Technology (2010), 23

In this work we study, both experimentally and numerically, the self-heating of a bulk, large YBCO pellet of aspect ratio (thickness / diameter) ~ 0.4 subjected to a large AC magnetic field. To ensure ... [more ▼]

In this work we study, both experimentally and numerically, the self-heating of a bulk, large YBCO pellet of aspect ratio (thickness / diameter) ~ 0.4 subjected to a large AC magnetic field. To ensure accurate temperature measurements, the sample was placed in an experimental vacuum chamber to achieve a small and reproducible heat transfer coefficient between the superconductor and the cryogenic fluid. The temperature was measured at several locations on the sample surface during the self-heating process. The experimentally determined temperature gradients are found to be very small in this arrangement (< 0.2 K across the radius of the superconductor). The time-dependence of the average temperature T(t) is found to agree well with a theoretical prediction based on the one-dimensional (1-D) Bean model, assuming a uniform temperature in the sample. A 2-D magneto-thermal model was also used to determine the space and time-dependent temperature distribution T(r, z, t) during the application of the AC field. The losses in the bulk pellet were determined using an algorithm based on the numerical method of Brandt, which was combined with a heat diffusion algorithm implemented using a finite-difference method. The model is shown to be able to reproduce the main trends of the observed temperature evolution of the bulk sample during a self-heating process. Finally, the 2-D model is used to study the effect of a non-uniform distribution of critical current density Jc(r, z) on the losses within the bulk superconductor. [less ▲]

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See detailMagneto-thermal phenomena in bulk high temperature superconductors subjected to applied AC magnetic fields
Vanderbemden, Philippe ULg; Laurent, Philippe ULg; Fagnard, Jean-François ULg et al

in Superconductor Science and Technology (2010), 23

In the present work we study, both theoretically and experimentally, the temperature increase in a bulk high temperature superconductor subjected to applied AC magnetic fields of large amplitude. We ... [more ▼]

In the present work we study, both theoretically and experimentally, the temperature increase in a bulk high temperature superconductor subjected to applied AC magnetic fields of large amplitude. We calculate analytically the equilibrium temperatures of the bulk sample as a function of the experimental parameters using a simple critical state model for an infinitely long type-II superconducting slab or cylinder. The results show the existence of a limit heat transfer coefficient (AUlim) separating two thermal regimes with different characteristics. The theoretical analysis predicts a ‘forbidden’ temperature window within which the temperature of the superconductor can never stabilize when the heat transfer coefficient is small. In addition, we determine analytical expressions for two threshold fields Htr1 and Htr2 characterizing the importance of magneto-thermal effects and show that a thermal runaway always occurs when the field amplitude is larger than Htr2. The theoretical predictions of the temperature evolution of the bulk sample during a self-heating process agree well with the experimental data. The simple analytical study presented in this paper enables order of magnitude thermal effects to be estimated for simple superconductor geometries under applied AC magnetic fields and can be used to predict the influence of experimental parameters on the self-heating characteristics of bulk type-II superconductors. [less ▲]

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See detailShielding efficiency and E(J) characteristics measured on large melt cast Bi-2212 hollow cylinders in axial magnetic fields
Fagnard, Jean-François ULg; Elschner, Steffen; Bock, Joachim et al

in Superconductor Science and Technology (2010), 23

We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets can also act as efficient magnetic shields. The magnetic screening properties under an axial DC magnetic field are ... [more ▼]

We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets can also act as efficient magnetic shields. The magnetic screening properties under an axial DC magnetic field are characterized at several temperatures below the liquid nitrogen temperature (77 K). Two main shielding properties are studied and compared with those of Bi-2223, a material that has been considered in the past for bulk magnetic shields. The first property is related to the maximum magnetic flux density that can be screened, Blim; it is defined as the applied magnetic flux density below which the field attenuation measured at the centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall thickness of 5 mm and a large ratio of length over radius, Blim is evaluated to 1 T at T = 10 K. This value largely exceeds the Blim value measured at the same temperature on similar tubes of Bi-2223. The second shielding property that is characterized is the dependence of Blim with respect to variations of the sweep rate of the applied field, dBapp/dt. This dependence is interpreted in terms of the power law E = Ec(J/Jc)^n and allows us to determine the exponent n of this E(J) characteristics for Bi-2212. The characterization of the magnetic field relaxation involves very small values of the electric field. This gives us the opportunity to experimentally determine the E(J) law in an unexplored region of small electric fields. Combining these results with transport and AC shielding measurements, we construct a piecewise E(J) law that spans over 8 orders of magnitude of the electric field. [less ▲]

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See detailPulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface
Lousberg, Grégory ULg; Fagnard, Jean-François ULg; Haanappel, Evert et al

in Superconductor Science and Technology (2009), (22), 125026

We present a method for characterizing the propagation of the magnetic flux in an artificially drilled bulk high-temperature superconductor (HTS) during pulsed-field magnetization. As the magnetic pulse ... [more ▼]

We present a method for characterizing the propagation of the magnetic flux in an artificially drilled bulk high-temperature superconductor (HTS) during pulsed-field magnetization. As the magnetic pulse penetrates the cylindrical sample, the magnetic flux density is measured simultaneously in 16 holes by means of microcoils that are placed across the median plane, i.e. at an equal distance from the top and bottom surfaces, and close to the surface of the sample. We discuss the time evolution of the magnetic flux density in the holes during a pulse and measure the time taken by the external magnetic flux to reach each hole. Our data show that the flux front moves faster in the median plane than on the surface when penetrating the sample edge; it then proceeds faster along the surface than in the bulk as it penetrates the sample further. Once the pulse is over, the trapped flux density inside the central hole is found to be about twice as large in the median plane as on the surface. This ratio is confirmed by modelling. [less ▲]

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See detailMagnetic shielding properties of high- Tc superconducting hollow cylinders: model combining experimental data for axial and transverse magnetic field configurations
Fagnard, Jean-François ULg; Dirickx, Michel; Ausloos, Marcel ULg et al

in Superconductor Science and Technology (2009), 22(10), 10500210

Magnetic shielding efficiency was measured on high- Tc superconducting hollow cylinders subjected to either an axial or a transverse magnetic field in a large range of field sweep rates, dBapp/dt. The ... [more ▼]

Magnetic shielding efficiency was measured on high- Tc superconducting hollow cylinders subjected to either an axial or a transverse magnetic field in a large range of field sweep rates, dBapp/dt. The behaviour of the superconductor was modelled in order to reproduce the main features of the field penetration curves by using a minimum number of free parameters suitable for both magnetic field orientations. The field penetration measurements were carried out on Pb-doped Bi-2223 tubes at 77 K by applying linearly increasing magnetic fields with a constant sweep rate ranging between 10 uT s[?]1 and 10 mT s[?]1 for both directions of the applied magnetic field. The experimental curves of the internal field versus the applied field, Bin(Bapp), show that, at a given sweep rate, the magnetic field for which the penetration occurs, Blim, is lower for the transverse configuration than for the axial configuration. A power law dependence with large exponent, n', is found between Blim and dBapp/dt. The values of n' are nearly the same for both configurations. We show that the main features of the curves Bin(Bapp) can be reproduced using a simple 2D model, based on the method of Brandt, involving a E(J) power law with an n-exponent and a field-dependent critical current density, Jc(B), (following the Kim model: Jc = Jc0(1+B/B1)[?]1). In particular, a linear relationship between the measured n'-exponents and the n-exponent of the E(J) power law is suggested by taking into account the field dependence of the critical current density. Differences between the axial and the transverse shielding properties can be simply attributed to demagnetizing fields. [less ▲]

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See detailNumerical simulation of the magnetization of high-temperature superconductors: a 3D finite element method using a single time-step iteration
Lousberg, Grégory ULg; Ausloos, Marcel ULg; Geuzaine, Christophe ULg et al

in Superconductor Science and Technology (2009), 22

In this paper, we report progress towards a 3D finite element model for the magnetization of a high-temperature superconductor (HTS): we suggest a method that takes into account a power law conductivity ... [more ▼]

In this paper, we report progress towards a 3D finite element model for the magnetization of a high-temperature superconductor (HTS): we suggest a method that takes into account a power law conductivity and demagnetization effects, while neglecting the effects associated with currents that are not perpendicular to the local magnetic induction. We consider samples that are subjected to a uniform magnetic field varying linearly with time. Their magnetization is calculated by means of a weak formulation in the magnetostatic approximation of the Maxwell equations (A–φ formulation). An implicit method is used for the temporal resolution (backward Euler scheme) and is solved with the open source solver GetDP. Fixed point iterations are used to deal with the power law conductivity of HTS. The finite element formulation is validated for an HTS tube with large n value by comparing with results obtained with other well-established methods. We show that carrying out the calculations with a single time-step (as opposed to many small time-steps) produces results with excellent accuracy in a drastically reduced simulation time. The numerical method is extended to the study of the trapped magnetization of cylinders that are drilled with different arrays of columnar holes arranged parallel to the cylinder axis. [less ▲]

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See detailMeasurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor
Lousberg, Grégory ULg; Fagnard, Jean-François ULg; Noudem, Jacques G et al

in Superconductor Science and Technology (2009), 22

We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic properties in the volume of the sample. The sample is subjected to an AC magnetic flux with a density ranging from 30 ... [more ▼]

We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic properties in the volume of the sample. The sample is subjected to an AC magnetic flux with a density ranging from 30 to 130 mT and the flux in the superconductor is probed by miniature coils inserted in the holes. In a given hole, three different penetration regimes can be observed: (i) the shielded regime, where no magnetic flux threads the hole; (ii) the gradual penetration regime, where the waveform of the magnetic field has a clipped sine shape whose fundamental component scales with the applied field; and (iii) the flux concentration regime, where the waveform of the magnetic field is nearly a sine wave, with an amplitude exceeding that of the applied field by up to a factor of two. The distribution of the penetration regimes in the holes is compared with that of the magnetic flux density at the top and bottom surfaces of the sample, and is interpreted with the help of optical polarized light micrographs of these surfaces. We show that the measurement of the magnetic field inside the holes can be used as a local characterization of the bulk magnetic properties of the sample. [less ▲]

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See detailPinning centers produced by magnetic microstructures
Silhanek, Alejandro ULg; Van de Vondel, J.; Leo, A. et al

in Superconductor Science and Technology (2009), 22(3),

We investigate the flux pinning and dynamic properties of superconducting vortices in an Al film with an array of magnetic bars deposited on top. The dimensions of each bar are chosen in such a way that ... [more ▼]

We investigate the flux pinning and dynamic properties of superconducting vortices in an Al film with an array of magnetic bars deposited on top. The dimensions of each bar are chosen in such a way that they host a single magnetic domain. These micromagnets are distributed periodically in a rectangular array with 0.5 mu m separation parallel to the longest side of the bars and displaced laterally by a distance w. We show that, for w > Lambda, where Lambda is the effective field penetration depth, the pinning strength is almost independent of w whereas the critical temperature at zero field, T(c)(0), decreases as similar to w(-1). For w < Lambda the opposite behavior is observed, i.e. T(c)(0) seems to saturate to a nearly w-independent value and the transition from large to small w is accompanied by a large suppression of the critical current j(c) together with a clear change in the shape of the current-voltage characteristics. In particular, the substantial weakening of the pinning potential for w < Lambda gives rise to an unexpected flux flow response. The field evolution of this regime allows us to determine whether the magnetic bars induce vortex-antivortex pairs in the system. The present findings suggest that practical application of magnetic pinning centers are restricted to low field values. [less ▲]

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See detailNucleation of superconductivity and vortex matter in superconductor-ferromagnet hybrids
Aladyshkin, A Yu; Silhanek, Alejandro ULg; Gillijns, W. et al

in Superconductor Science and Technology (2009), 22(5),

The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially separated conventional low-temperature ... [more ▼]

The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially separated conventional low-temperature superconductors (S) and ferromagnets (F), are reviewed. Since the superconducting and ferromagnetic parts are assumed to be electrically insulated, no proximity effect is present and thus the interaction between both subsystems is through their respective magnetic stray fields. Depending on the temperature range and the value of the external field H(ext), different behavior of such S/F hybrids is anticipated. Rather close to the superconducting phase transition line, when the superconducting state is only weakly developed, the magnetization of the ferromagnet is solely determined by the magnetic history of the system and it is not influenced by the field generated by the supercurrents. In contrast to that, the nonuniform magnetic field pattern, induced by the ferromagnet, strongly affects the nucleation of superconductivity, leading to an exotic dependence of the critical temperature T(c) on H(ext). Deeper in the superconducting state the effect of the screening currents cannot be neglected anymore. In this region of the phase diagram T-H(ext) various aspects of the interaction between vortices and magnetic inhomogeneities are discussed. In the last section we briefly summarize the physics of S/F hybrids when the magnetization of the ferromagnet is no longer fixed but can change under the influence of the superconducting currents. As a consequence, the superconductor and ferromagnet become truly coupled and the equilibrium configuration of this 'soft' S/F hybrid requires rearrangements of both superconducting and ferromagnetic characteristics, as compared with 'hard' S/F structures. [less ▲]

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See detailBulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux?
Lousberg, Grégory ULg; Ausloos, Marcel ULg; Vanderbemden, Philippe ULg et al

in Superconductor Science and Technology (2008), 21

Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat exchange with the cooling liquid. However, drilling holes also reduces the amount of magnetic flux that can be ... [more ▼]

Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat exchange with the cooling liquid. However, drilling holes also reduces the amount of magnetic flux that can be trapped in the sample. In this paper, we use the Bean model to study the magnetization and the current line distribution in drilled samples, as a function of the hole positions. A single hole perturbs the critical current flow over an extended region that is bounded by a discontinuity line, where the direction of the current density changes abruptly. We demonstrate that the trapped magnetic flux is maximized if the centre of each hole is positioned on one of the discontinuity lines produced by the neighbouring holes. For a cylindrical sample, we construct a polar triangular hole pattern that exploits this principle; in such a lattice, the trapped field is ∼20% higher than in a squared lattice, for which the holes do not lie on discontinuity lines. This result indicates that one can simultaneously enhance the oxygen annealing, the heat transfer and maximize the trapped field. [less ▲]

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See detailRemagnetization of bulk high-temperature superconductors subjected to crossed and rotating magnetic fields
Vanderbemden, Philippe ULg; Hong, Z.; Coombs, T. A. et al

in Superconductor Science and Technology (2007, September), 20(9 Sp. Iss. SI), 174-183

Bulk melt-processed Y-Ba-Cu-O (YBCO) has significant potential for a variety of high-field permanent-magnet-like applications, such as the rotor of a brushless motor. When used in rotating devices of this ... [more ▼]

Bulk melt-processed Y-Ba-Cu-O (YBCO) has significant potential for a variety of high-field permanent-magnet-like applications, such as the rotor of a brushless motor. When used in rotating devices of this kind, however, the YBCO can be subjected to both transient and alternating magnetic fields that are not parallel to the direction of magnetization and which have a detrimental effect on the trapped field. These effects may lead to long-term decay of the magnetization of the bulk sample. In the present work, we analyze both experimentally and numerically the remagnetization process of a melt-processed YBCO single domain that has been partially demagnetized by a magnetic field applied orthogonal to the initial direction of trapped flux. Magnetic torque measurements are used as a tool to probe changes in the remanent magnetization during various sequences of applied field. The application of a small magnetic field between the transverse cycles parallel to the direction of original magnetization results in partial remagnetization of the sample. Rotating the applied field, however, is found to be much more efficient at remagnetizing the bulk material than applying a magnetizing field pulse of the same amplitude. The principal features of the experimental data can be reproduced qualitatively using a two-dimensional finite-element numerical model based on an E-J power law. Finally, the remagnetization process is shown to result from the complex modification of current distribution within the cross-section of the bulk sample. [less ▲]

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See detailField penetration into hard type-II superconducting tubes: effects of a cap, a non-superconducting joint, and non-uniform superconducting properties
Denis, Samuel ULg; Dirickx, Michel ULg; Vanderbemden, Philippe ULg et al

in Superconductor Science and Technology (2007), 20(5), 418-427

Using the numerical method of Brandt (1998 Phys. Rev. B 58 6506), we study the penetration of a uniform magnetic field that is gradually applied parallel to the axis of finite type-II superconducting ... [more ▼]

Using the numerical method of Brandt (1998 Phys. Rev. B 58 6506), we study the penetration of a uniform magnetic field that is gradually applied parallel to the axis of finite type-II superconducting tubular samples with strong pinning. This study is carried out in view of designing low-frequency magnetic shields by exploiting the diamagnetic properties of type-II superconductors. First, we compare the field penetration into open and closed tubes. For long tubes (length larger than three times the outer diameter), we show that a cap weakly affects the maximum magnetic induction that can be shielded, but greatly increases the region over which the field is nearly uniform. When the length of the tube is shorter, both the maximum shieldable magnetic induction and the uniformity of the field attenuation are enhanced by closing the tube. We also show that making a hole in the cap, which is often necessary for applications, does not greatly affect the shielding properties provided the diameter of the hole is small compared to that of the tube (hole diameter smaller than a quarter of the outer tube diameter). In view of designing large size magnetic shields, superconducting tubes of finite size need to be joined together. We therefore examine in a second part how the presence of a non-superconducting joint between the tubes affects the shielding efficiency. It is shown that the effect of a joint depends upon its position along the tube axis and strongly increases with its size. Third, we study how non-uniform superconducting properties affect the shielding capabilities. [less ▲]

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See detailMagnetic shielding properties of high-temperature superconducting tubes subjected to axial fields
Denis, Samuel ULg; Dusoulier, Laurent; Dirickx, Michel ULg et al

in Superconductor Science and Technology (2007), 20(3), 192-201

We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a ... [more ▼]

We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a function of the dimensions of the tube, the magnitude of the applied field and the frequency. These results are explained quantitatively by employing the method of Brandt ( 1998 Phys. Rev. B 58 6506) with a Jc( B) law appropriate for a polycrystalline material. Specifically, we observe that the applied field can sweep into the central region either through the thickness of the shield or through the opening ends, the latter mechanism being suppressed for long tubes. For the first time, we systematically detail the spatial variation of the shielding factor ( the ratio of the applied field over the internal magnetic field) along the axis of a high-temperature superconducting tube. The shielding factor is shown to be constant in a region around the centre of the tube, and to decrease as an exponential in the vicinity of the ends. This spatial dependence comes from the competition between two mechanisms of field penetration. The frequency dependence of the shielding factor is also discussed and shown to follow a power law arising from the finite creep exponent n. [less ▲]

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See detailEBSD study on YBCO textured bulk samples: correlation between crystal growth and 'microtexture'
Grossin, D.; Henrist, Catherine ULg; Mathieu, J. P. et al

in Superconductor Science and Technology (2006), 19(2), 190-199

This work describes an electron backscattered diffraction (EBSD) study of the perovskite-derived structures YBa2Cu3O7-delta. After having pointed out the difficulties of EBSD analyses in resolving the ... [more ▼]

This work describes an electron backscattered diffraction (EBSD) study of the perovskite-derived structures YBa2Cu3O7-delta. After having pointed out the difficulties of EBSD analyses in resolving the orientations of these pseudo-cubic structures, various YBaCuO bulk samples are analysed and the correlation between the microstructure, crystal growth and global texture, determined by neutron diffraction, is carried out. Homogeneous 'microtexture' with small subdomain misorientation of 12 degrees are measured for YBCO top seeding melt textured growth (TSMTG) samples. YBCO perforated samples also exhibit misoriented subdomains, giving rise to a heterogeneous 'microtexture' correlated to the YBCO growth front and to the pattern used for the perforating. [less ▲]

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See detailStudy by Hall probe mapping of the trapped flux modification produced by local heating in YBCOHTS bulks for different surface/volume ratios
Laurent, Philippe ULg; Mathieu, Jean-Philippe; Mattivi, Brice et al

in Superconductor Science and Technology (2005), 18(8), 1047-1053

The aim of this report is to compare the trapped field distribution under a local heating created at the sample edge for different sample morphologies. Hall probe mappings of the magnetic induction ... [more ▼]

The aim of this report is to compare the trapped field distribution under a local heating created at the sample edge for different sample morphologies. Hall probe mappings of the magnetic induction trapped in YBCO bulk samples maintained out of thermal equilibrium were performed on YBCO bulk single domains, YBCO single domains with regularly spaced hole arrays, and YBCO superconducting foams. The capability of heat draining was quantified by two criteria: the average induction (B) decay and the size of the thermally affected zone caused by a local heating of the sample. Among the three investigated sample shapes, the drilled single domain displays a trapped induction which is weakly affected by the local heating while displaying a high trapped field. Finally, a simple numerical modelling of the heat flux spreading into a drilled sample is used to suggest some design rules about the hole configuration and their size. [less ▲]

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