References of "Fagnard, Jean-François"
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See detailMagnetic shielding of open and semi-closed bulk superconductor tubes: the role of a cap
Wéra, Laurent; Fagnard, Jean-François ULiege; Hogan, Kevin et al

in IEEE Transactions on Applied Superconductivity (2019), 29(3), 6801109

In this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap ... [more ▼]

In this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap placed against its extremity. The results are compared with those obtained on a bulk large grain Y-Ba-Cu-O (YBCO) tube produced by buffer-aided top seeded melt growth. In this process, the disk-shaped pellet and the tubular sample are grown together, resulting in a tube naturally closed at one extremity. The field to be shielded is either parallel or perpendicular to the main axis of the tube. The experimental results are compared with the results of finite element numerical modeling carried out either in two dimensions (for the axial configuration) or three dimensions (for the transverse configuration). In the axial configuration, the results show that the shielded volume can be enhanced easily by increasing the thickness of the cap. In the transverse configuration, the results show the critical role played by the superconducting current loops flowing between the tube and the cap for magnetic shielding. If the tube and the cap are separated by a non-superconducting joint or air gap, the presence of a cap leads only to a small improvement of the transverse shielding factor, even for a configuration where the gap between the cap and the tube contains a 90° bend. The cap leads to a significant increase in the transverse shielding when the cap and the tube are naturally grown in the same process, i.e., made of a continuous superconducting material. The experimental results can be reproduced qualitatively by 3-D numerical modeling. [less ▲]

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See detailBulk superconducting tube subjected to the stray magnetic field of a solenoid
Hogan, Kevin ULiege; Fagnard, Jean-François ULiege; Wera, Laurent ULiege et al

in Superconductor Science and Technology (2018), 31

Hard type-II hollow superconductors are well suited for low frequency magnetic shielding. The properties and performances of superconducting magnetic shields subjected to homogeneous magnetic fields have ... [more ▼]

Hard type-II hollow superconductors are well suited for low frequency magnetic shielding. The properties and performances of superconducting magnetic shields subjected to homogeneous magnetic fields have been extensively discussed in the literature. In the present work, we investigate the magnetic shielding and the penetration of magnetic flux in a bulk high temperature superconducting tube subjected to the inhomogeneous fringe field of a solenoidal coil. Thanks to a bespoke microdisplacement measurement system, we measure the magnetic field distribution around the tube. We develop a full 3D finite element model based on an H formulation to understand the flux penetration mechanisms and predict the shape of the current loops. Using constitutive law parameters obtained from previous independent experiments, our model is found to be in excellent agreement with the measurements. We discuss how to assess the degree of inhomogeneity of the magnetic field and show that, in our case study, the field can be treated as the magnetic field of an equivalent magnetic dipole. We also show that some features of the flux penetration in inhomogeneous field can be also observed when the tube is subjected to an oblique homogeneous magnetic field, which offers a better understanding of the shielding current density distribution inside the shield. Finally, we discuss the magnetic field concentration occurring around the shield for different magnetic field configurations. In particular, we show that the extremities of the tube on the side not facing the magnetic field source experience the highest flux concentration. [less ▲]

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See detailMesures magnétiques : comment concevoir un magnétomètre adapté aux échantillons de grande taille ?
Vanderbemden, Philippe ULiege; Philippe, Matthieu; Wéra, Laurent et al

Conference (2018, October 18)

Les magnétomètres commerciaux traditionnels sont adaptés pour mesurer le moment magnétique d'échantillons de taille relativement réduite, en général < 1cm³. Pour certains matériaux cependant, il est ... [more ▼]

Les magnétomètres commerciaux traditionnels sont adaptés pour mesurer le moment magnétique d'échantillons de taille relativement réduite, en général < 1cm³. Pour certains matériaux cependant, il est désirable de pouvoir déterminer le moment magnétique d'échantillons plus volumineux de manière non-destructive; c'est le cas notamment des supraconducteurs massifs utilisés comme aimant permanents. Dans ce cas, il est nécessaire de revoir la conception de l'appareillage dans son ensemble, et notamment ta taille des bobines de détection. Dans cet exposé, nous montrerons comment dimensionner les bobines de détection d'un magnétomètre à extraction et comment traiter le signal mesuré de manière à déterminer aussi précisément que possible le moment magnétique d'un échantillon, potentiellement de grande taille. Nous envisagerons également le problème lié à la forme particulière (rapport d'aspect) de l'échantillon sur le signal mesuré. Nos propos seront illustrés par les résultats expérimentaux obtenus sur un magnétomètre conçu sur mesure pour les échantillons de taille importante. Reference : R. Egan et al., "A flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors." Rev Sci Instrum. (2015) 86(2), 025107. doi: 10.1063/1.4907903. [less ▲]

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See detailFEM Simulations of Crossed-field Effects in Bulk Superconducting Trapped-field Permanent Magnets
Vanderheyden, Benoît ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Conference (2018, April 30)

Bulk (RE)BCO samples can be used as efficient permanent magnets achieving trapped flux densities of a few teslas at 77 K. However, their magnetization decays over time when subjected to crossed fields ... [more ▼]

Bulk (RE)BCO samples can be used as efficient permanent magnets achieving trapped flux densities of a few teslas at 77 K. However, their magnetization decays over time when subjected to crossed fields (with a direction perpendicular to the initially trapped flux density). Such parasitic field may be generated in synchronous motors as stray fields. In this talk, I summarize the recent work carried out in our group at the University of Liege to better understand the crossed-field effects. Under a magnetic field perturbation with a non-vanishing crossed field, the superconductor magnetization is modified due to a rearrangement of currents. We have developed finite-element models predicting the time evolution of the current distribution, on the basis of an A-V formulation implemented in the open-source software GetDP and Gmsh. The construction of the models is discussed, including the time-step scheme used, the choice for the mesh and convergence issues. It is demonstrated how the redistribution of currents under a transverse field can be studied with either 3D models or computationally simpler 2D models. The crossed-field effects are illustrated as a function of the amplitude of the perturbations, their waveform, and the variation of their direction over time (purely transverse vs rotation in space). Hybrid structures made of superconductors and soft ferromagnetic materials are also investigated. For the case of a soft ferromagnetic disk put on top of a superconducting pellet, it is demonstrated that the ferromagnetic material helps in both increasing the initial trapped field and in reducing the crossed-field effects. Different hybrid structures are discussed in order to find a compromise between the maximum level of induction which can be reached and the improved protection against crossed-field effects. Potential extensions of the numerical models are also discussed. [less ▲]

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See detailThe influence of an initial trapped field on the magnetic shielding performance of bulk high-temperature superconducting tubes
Wéra, Laurent; Fagnard, Jean-François ULiege; Hogan, Kevin et al

in Miryala, Muralidhar; Koblischka, Michael (Eds.) High-Temperature Superconductors: Occurrence, Synthesis and Applications (2018)

Type-II superconducting hollow cylinders can act as efficient passive magnetic shields. Unlike in ferromagnetic materials, the magnetic shielding mechanism in a superconductor occurs through persistent ... [more ▼]

Type-II superconducting hollow cylinders can act as efficient passive magnetic shields. Unlike in ferromagnetic materials, the magnetic shielding mechanism in a superconductor occurs through persistent currents flowing on the macroscopic scale. When a superconducting tube is used to shield a sequence of magnetic fields with different orientations, magnetic shielding performance levels are likely to be degraded because of the superconducting currents that are trapped in the superconductor. In this chapter we study experimentally the magnetic shielding configurations where an external magnetic field is applied first along one direction and a second field is subsequently applied along another (perpendicular) direction. In particular, we focus on the effect of an axial trapped field on the transverse shielding performance and on the effect of a transverse trapped field on the axial or the transverse shielding performance. Finally, we show how the pristine state of the tube can be restored by an appropriate magnetic field sequence, i.e., without heating up the shield above its critical temperature Tc. [less ▲]

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See detailStudy of buoyancy driven heat transport in silicone oils and in liquid nitrogen in view of cooling applications
Satpathy, Kamalakanta ULiege; Duchesne, Alexis; Dubois, Charles et al

in International Journal of Heat and Mass Transfer (2018), 118

Motivated by applications for cooling superconducting pellets with liquid nitrogen, we consider a source with a fixed heating rate per unit volume, immersed in a liquid pool and cooled through natural ... [more ▼]

Motivated by applications for cooling superconducting pellets with liquid nitrogen, we consider a source with a fixed heating rate per unit volume, immersed in a liquid pool and cooled through natural convection. In one recent experimental investigation (Dubois et al., 2016) carried on silicone oils and liquid nitrogen, we have demonstrated that the velocity field satisfies specific scaling laws with respect to the temperature increase in the liquid pool. In this work, we pursue the analysis by modeling the heat transfer in a parallelepiped enclosure for a steady laminar flow regime. The Navier-Stokes equations are solved using a finite volume approach to obtain the detailed three-dimensional flow and heat transfer characteristics. A quantitative analysis of the velocity field over the temperature field shows that the experimental power laws are reproduced in simulations. Following Dubois and Berge (1978), a theoretical law originally introduced in the context of the classical Rayleigh-Bénard experiment is shown to be satisfied in the simulations over a wide range of Rayleigh numbers (Ra), assuming the definition of the characteristic convection length is adapted to the investigated geometry. Moreover, the simulations are shown to correctly reproduce the main features of the flow, including the characteristic convection length, for different heater lengths. [less ▲]

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See detailMagnetic shielding of open and semi-closed tubes made of bulk superconductors : the role of a cap
Vanderbemden, Philippe ULiege; Wera, Laurent ULiege; Hogan, Kevin ULiege et al

Conference (2017, December 11)

Amongst the range of prospective applications for bulk superconductors, the shielding of high magnetic fields at low frequency is extremely promising. Unlike conventional high permeability ferromagnetic ... [more ▼]

Amongst the range of prospective applications for bulk superconductors, the shielding of high magnetic fields at low frequency is extremely promising. Unlike conventional high permeability ferromagnetic materials which exhibit a relatively low saturation magnetization (e.g. µ0Msat ~ 0.75 T for mumetal), superconductors do not suffer from this drawback. They are therefore well suited to shield magnetic flux densities above 1 tesla. This can be achieved by hollow bulk superconductors of cylindrical geometry. The shielding efficiency and shielded volume are both strongly improved when the hollowed tube is closed at one extremity. As an example, we showed recently [1] that YBa2Cu3O7 (YBCO) tubes of ~ 10 mm inner diameter closed at one extremity and engineered by a buffer-aided top seeded melt growth fabrication process were able to attenuate axial flux densities of 1.5 T by a factor of 100 at 20 K. Such record performances are partly due to the presence of a cap at one extremity. The aim of the present work is to examine further and understand the influence of a cap on the shielding properties for various configurations. In this communication we investigate the shielding properties of hollow and semi-closed bulk superconducting tubes at 77 K. For the semi-closed configuration, we compare situations the cap is either placed against the extremity (with a small controllable gap between them) or welded naturally. The field to be shielded is either parallel or perpendicular to the main axis of the tube. Both YBCO tubes produced by buffer-aided top seeded melt growth and commercial Bi-2223 tubes are studied. The experimental results are compared to finite element numerical modelling carried out either in 2D (for the axial configuration) or 3D (for the transverse configuration). In the axial configuration, the results show that a small gap between the tube and the cap is not detrimental to the shielding properties. We also show that the shielded volume can be easily enhanced by increasing the thickness of the cap or its critical current density. In the transverse configuration, the results point out the crucial role of the superconducting joint between the tube and the cap. The need for a superconducting joint depends therefore on the main direction of the magnetic field to be shielded. References: [1] L. Wéra. et al., IEEE. Trans. Appl. Supercond. 27 (2017) 6800305 [less ▲]

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See detailInvestigation of the shielding properties of a Bi-2212 bulk superconducting tube subjected to an inhomogeneous magnetic field at various temperatures
Hogan, Kevin ULiege; Wera, Laurent ULiege; Fagnard, Jean-François ULiege et al

Poster (2017, December 11)

Magnetic shielding with bulk superconductors is commonly studied under homogeneous magnetic fields. This configuration simplifies the analysis of the magnetic reaction of the superconductor as the applied ... [more ▼]

Magnetic shielding with bulk superconductors is commonly studied under homogeneous magnetic fields. This configuration simplifies the analysis of the magnetic reaction of the superconductor as the applied magnetic field is known precisely everywhere. In practical applications, however, the magnetic field to be screened is seldom homogeneous. Magnetic shields experience magnetic field gradients and flux lines curvatures that might lead to a deterioration of the shielding properties. In the present work, we investigate experimentally the magnetic shielding properties of a bulk superconducting Bi-2212 tube subjected to an inhomogeneous magnetic field. This study is carried out at various temperatures thanks to a bespoke variable temperature measurement system based on a cryocooler. A superconducting pancake coil, made of 1G superconducting tape (Bi-2223), generates the inhomogeneous magnetic field to be screened. The bulk superconducting tube is anchored thermally to a sample holder fixed in the vicinity of the cold head. A cryogenic Hall probe is placed at the center of the bulk Bi-2212 tube to characterize the shielding properties of the superconductor. With this system, we characterize the threshold induction, i.e. the maximal applicable magnetic field below which the shielding factor (ratio between the measured and applied field) drops below a threshold value, at various temperatures ranging from 60 K to 92 K. Then we show and discuss the effect of the sweep rate of the applied magnetic field on the threshold induction value. Our experimental results are confronted to experimental results obtained at 77 K in an axial homogeneous magnetic field. This allow us to emphasize the impact of the inhomogeneity of the applied magnetic field on the shielding properties. We finally point out the portions of the magnetic field shield experiencing the highest magnetic field. [less ▲]

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See detailStudies on convective cooling of cryogenic fluids towards superconducting applications
Satpathy, Kamalakanta ULiege; Dubois, Charles; Duchesne, Alexis et al

in WIT Transactions on Engineering Sciences (2017), 118

To understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form ... [more ▼]

To understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form of N-S equations is solved to obtain the detailed flow features through path line profiles, isotherm contours and velocity vectors. The effect of heater aspect ratio (x/L) on the rate of heat transfer is studied in terms of the average Nusselt number (Nuave). The results indicate that effective heat transfer enhancement occurs for a small heater length, resulting in an efficient cooling. Increasing the heater length will favor heat transfer through conduction over convection. The maximum temperature difference across the fluid and the velocity magnitude are found to decrease with heater length. 3D and 2D results are in agreement for short heater lengths, but vary for higher heater lengths, presumably due to the essential effect of the heater width. Further analysis on different types of coolant reveals a constant correlation between Nuave and the Rayleigh number (Ra), with Nuave ~ Ra^0.374. Benchmark validation for natural convection in a square enclosure is found to be satisfactory against the reported results. [less ▲]

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See detailModel for the response of superconducting/ferromagnetic structures subjected to crossed fields
Vanderheyden, Benoît ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2017, September 20)

Bulk (RE)BCO samples can be used as efficient permanent magnets with trapped magnetic flux densities of a few teslas at 77 K. However, their magnetization may decay over time when subjected to time ... [more ▼]

Bulk (RE)BCO samples can be used as efficient permanent magnets with trapped magnetic flux densities of a few teslas at 77 K. However, their magnetization may decay over time when subjected to time-varying parasitic fields perpendicular to the initial trapped flux density. In this work, we consider hybrid structures made of superconducting pellets and soft ferromagnetic rings or disks and investigate numerically how their magnetization varies under such crossed-field excitations. We consider a finite-element model in the A-V formulation, which is developed for both the full 3D configuration of the system and for a computationally lighter 2D approximation. The models provide the volumic distribution of current densities, the field lines, and the magnetization of the system, and the latter is compared to experimental measurements obtained with a bespoke magnetometer. A critical assessment is made outlining the advantages and limitations of the 2D approach. It is demonstrated that the ferromagnetic material helps in increasing the initial trapped field and in reducing the crossed-field effects. For the case of a single ferromagnetic disk put on top of the superconducting pellet, we study the dependence of the initial magnetization with respect to the disk thickness and the saturation field of the ferromagnetic material, and discuss the resulting optimal thickness. We also suggest different hybrid structures made of one superconducting pellet and one or several ferromagnetic disks or rings, and show numerically that a compromise is to be found between the maximum level of induction which can be reached and the improved protection against crossed-field effects. [less ▲]

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See detailNon-destructive investigation of hybrid of ferromagnet /(RE)BCO large grain bulks by flux extraction magnetometry and levitation force
Vanderbemden, Philippe ULiege; Philippe, Matthieu; Wera, Laurent ULiege et al

Conference (2017, July 10)

This work deals with bulk, large grain superconductors used as permanent magnet for rotating machines or levitation applications. It has recently been shown that the magnetic properties of bulk large ... [more ▼]

This work deals with bulk, large grain superconductors used as permanent magnet for rotating machines or levitation applications. It has recently been shown that the magnetic properties of bulk large grain superconductors can be improved easily by attaching a short section of a soft ferromagnetic material (F) to one of the faces of the bulk superconductor (S), thereby producing a hybrid F/S structure [1]. Here we investigate the contactless determination of the magnetic behavior of such structures using a recently constructed bespoke magnetometer based on the flux extraction technique [2]. This device allows magnetic moments as large as 1 Am² to be measured at 77 K and accommodates large bulk samples up to 20 mm diameter. This extends significantly the accessible measurement range of “off-the shelf” magnetometers. Unlike techniques based on recording the distribution of flux at the surface of the sample, the measured signal is representative of the superconducting currents flowing across the entire volume of the sample. In the present work we examine the properties of permanently magnetized superconductors and hybrid structures, and measure the irreversible demagnetization of these structures when they are subjected to magnetic field cycles that are not parallel to their magnetization. We also investigate the levitation behavior of hybrid structures subjected to the non-uniform field of a permanent magnet or a combination of permanent magnets used as guideway for levitation applications, and compare the results to those obtained with a bulk superconductor alone. This abstract is for the special session on bulk superconductivity. References: [1] Egan R. et al., Rev. Sci. Instrum. 86 (2015) 025107 [2] M. P. Philippe et al., Supercond. Sci. Technol. 28 (2015) 095008 Acknowledgments: We greatly acknowledge Nippon Steel & Sumitomo Metal Corporation for providing bulk large grain GdBa2Cu3O7 (GdBCO) samples. [less ▲]

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See detailMagnetic Shielding Above 1 T at 20 K With Bulk, Large Grain YBCO Tubes Made by Buffer-aided Top Seeded Melt Growth
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Kumar, Numbury Devendra et al

in IEEE Transactions on Applied Superconductivity (2017), 27(4),

YBCO tubes of ~ 10 mm diameter closed at one extremity were engineered by a Buffer-Aided Top Seeded Melt Growth fabrication process (BA-TSMG). These tubes can act as efficient “dc” magnetic shields and ... [more ▼]

YBCO tubes of ~ 10 mm diameter closed at one extremity were engineered by a Buffer-Aided Top Seeded Melt Growth fabrication process (BA-TSMG). These tubes can act as efficient “dc” magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K. Such performances are comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. Magnetic shielding measurements for open and closed tubes at 77 K also show that the presence of the cap improves substantially the shielding performance at the closed extremity since it reduces the penetration through the open end. This fabrication technique is extremely promising for shielding “dc” stray fields generated by HTS magnets operated in a temperature range obtained by cryocoolers, liquid hydrogen (20 K) or liquid neon (27 K). [less ▲]

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See detailEffect of Ag substitution on structural, magnetic and magnetocaloric properties of Pr0.6Sr0.4– xAgxMnO3 manganites
Thaljaoui, Rachid; Pękała, Marek; Fagnard, Jean-François ULiege et al

in Journal of Rare Earths (2017), 35(9), 875-882

A systematic investigation on the structural, magnetic and magnetocaloric properties of Pr0.6Sr0.4–xAgxMnO3 (x=0.05 and 0.1) manganites was reported. Rietveld refinements of the X-ray diffraction patterns ... [more ▼]

A systematic investigation on the structural, magnetic and magnetocaloric properties of Pr0.6Sr0.4–xAgxMnO3 (x=0.05 and 0.1) manganites was reported. Rietveld refinements of the X-ray diffraction patterns confirmed that all samples were single phase and crystallized in the orthorhombic structure with Pnma space group. Magnetic measurements in a magnetic applied field of 0.01 T revealed that the ferromagnetic-paramagnetic transition temperature TC decreased from about 293 to 290 K with increasing silver content from x=0.05 to 0.1. The reported magnetocaloric entropy change and relative cooling power for both samples were considerably remarkable with a ΔSmax value of 1.9 J/(kg·K) and maximum RCP values of 100 J/kg, under a magnetic field change (Δµ0H) equal to 1.8 T. The analysis of the universal curves gave an evidence of a second order magnetic transition for the studied samples. The magnetic field influence on both the magnetic entropy change and the relative cooling power was also studied and discussed. © 2017 The Chinese Society of Rare Earths [less ▲]

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See detailBehaviour of bulk superconducting trapped field magnets subjected to magnetic fields with a transverse component
Vanderbemden, Philippe ULiege; Caps, Hervé ULiege; Vanderheyden, Benoît ULiege et al

Conference (2016, November 10)

Bulk superconductors can be used as trapped field magnets with unprecedented performances. Such trapped field magnets find a variety of engineering applications, in particular for brushless rotating ... [more ▼]

Bulk superconductors can be used as trapped field magnets with unprecedented performances. Such trapped field magnets find a variety of engineering applications, in particular for brushless rotating machines and levitation devices. Compared to conventional permanent magnets, however, the magnetic field generated by a superconducting trapped field magnet is much more sensitive to the presence of an external field that is not perfectly aligned with the superconductor magnetic moment, i.e. when the external field has a transverse component. A situation where the superconductor experiences a magnetic field with a transverse component is likely to happen several times during running operation of the device. As an example, in a “trapped flux” synchronous machine, any variation of the load of the shaft may lead to a slight misalignment of the magnetic moment of the superconductor (placed in the rotor) with respect to the rotating field (generated by the stator). In such a situation, the superconductor is subjected to a rotating field, and the situation is likely to be different from a situation where the superconductor is subjected to a pure transverse field. In the present work, we aim at examining different scenarios (i.e. transverse or rotating) where the trapped field magnet is subjected to a transverse component. These configurations are examined both experimentally and numerically. The numerical model is used to determine the distribution of currents in the cross-section of the superconductor. We examine the conditions where the rotating field leads to a partial remagnetization of the sample and is therefore less detrimental that the application of a pure transverse field. We also show that the magnetic moment of the sample may behave differently that the field at the surface of the superconductor. Finally, we investigate the behavior of structures where the superconductor is attached to a thin ferromagnetic disk, and show the beneficial effect of such hybrid structures compared to the superconductor alone. Acknowledgements : The research was funded through the University of Liège (ULg) and an ARC grant for Concerted Research Actions, financed by the French Community of Belgium (Wallonia-Brussels Federation), under reference ARC 11/16-03. We greatly acknowledge M. Morita, S. Nariki and H. Teshima from Nippon Steel & Sumitomo Metal Corporation, Futtsu, Chiba. [less ▲]

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See detailBulk high-temperature superconducting hollow cylinders used for magnetic shielding: effect of an initial trapped field on the shielding performances.
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2016, November 09)

Bulk High-Temperature Superconductors (HTS) can be used as efficient passive magnetic shields with performances exceeding those of conventional ferromagnetic materials. The most common geometry for a ... [more ▼]

Bulk High-Temperature Superconductors (HTS) can be used as efficient passive magnetic shields with performances exceeding those of conventional ferromagnetic materials. The most common geometry for a superconducting shield is a bulk hollow cylinder. In this work, we investigate the magnetic shielding properties when the tube is not in the virgin state. Such a situation happens in practice when the tube has been initially subjected to a first magnetic field. The induced superconducting currents that are flowing in the tube generate a trapped magnetic moment which may deteriorate the shielding of a second field. Here we study experimentally how an initial trapped moment affects the shielding of a magnetic field, when they are both perpendicular to each other. In particular, we focus on the effect of an axial trapped moment on the transverse shielding performances and on the effect of a transverse trapped field on the axial or the transverse shielding performances. Finally, we show how the pristine state of the tube can be restored when applying an external field of adequate amplitude. The consequence for practical applications is that it is possible to recover shielding properties similar to those obtained in the virgin state without heating up the shield above its critical temperature TC. [less ▲]

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See detailMagnetic shielding above 1 T at 20 K with bulk, large grain YBCO tubes made by Buffer-aided Top Seeded Melt Growth.
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Kumar, Nambury Devendra et al

Poster (2016, September 07)

A superconducting tube can be used as an efficient magnetic shield. The best shielding performance levels are usually achieved when the tube is closed at one or both extremities. The purpose of the ... [more ▼]

A superconducting tube can be used as an efficient magnetic shield. The best shielding performance levels are usually achieved when the tube is closed at one or both extremities. The purpose of the present work is to study experimentally the shielding performance of YBCO tubes obtained by Buffer-aided Top Seeded Melt Growth fabrication process (BA-TSMG). This fabrication process enables the tube to be closed at one extremity by a cap containing the seed and there is no air gap between the cap and the tube. The shielding effectiveness is characterized by two parameters: (i) the shielding factor, defined as the ratio between the applied magnetic induction and the local magnetic induction measured inside the shield, and (ii) the threshold induction, i.e. the applied magnetic induction above which a given value of the shielding factor cannot be achieved. The magnetic shielding performances of tubes with different geometry are characterized at 77 K in liquid nitrogen. Further magnetic shielding measurements are carried out on one tube at various temperatures ranging from 20 K to 77 K using a Physical Property Measurement System (PPMS). The tubes are subjected to a quasi-static (“DC”) uniform magnetic field. A Hall probe measures the axial component of the local magnetic induction along the axis of the tube as a function of the applied magnetic induction. In order to investigate how the cap prevents the magnetic flux from penetrating inside the tube, we also characterize open tubes where the cap is removed and compare their properties to those of closed tubes. Magnetic shielding measurements show that the threshold induction increases by a factor of 9 as temperature decreases from 77 K to 20 K. Measurements also show that the presence of the cap improves the shielding performance at the closed extremity of the order of 1000 as it reduces the penetration through the open end. Near the closed extremity, a threshold induction of 1.5 T was reached at 20 K. To our knowledge this threshold induction is the best value reported so far at 20 K, and is comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. These results give evidence that efficient magnetic shields can be obtained with this fabrication technique. [less ▲]

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See detailNon-destructive measurements of the volume magnetic behavior of large bulk GdBCO single domains and ferromagnet/superconductor structures
Vanderbemden, Philippe ULiege; Egan, Raphael; Morita, Mitsuru et al

Conference (2016, September 05)

The present work deals with magnetic measurements on bulk large grain GdBa2Cu3O7 (GdBCO) single domains using a recently constructed bespoke magnetometer [Rev. Sci. Instrum. 86 025107 (2015)]. The device ... [more ▼]

The present work deals with magnetic measurements on bulk large grain GdBa2Cu3O7 (GdBCO) single domains using a recently constructed bespoke magnetometer [Rev. Sci. Instrum. 86 025107 (2015)]. The device enables the measurement of magnetic moments as large as 1 Am^2 (1000 emu) on large bulk samples up to 20 mm diameter at 77 K. This extends significantly the accessible measurement range of “off-the shelf” magnetometers (e.g. SQUID, VSM). Unlike Hall probe mapping, the measured signal is representative of the superconducting currents flowing across the entire height of the sample. Since the device is based on a flux extraction technique and does not include any Hall probe, another advantage is that no gap between a Hall probe and the sample needs to be taken into account to interpret the results. In the present work we first show how the dimensions of the sensing coils and integration bounds are chosen in such a way the magnetometer is sensitive to dipolar magnetic moment of the sample, while being insensitive to moments of higher order. Next we study the relaxation of the trapped magnetic moment under various magnetization processes. The sensitivity of the device allows small variations of m (e.g. ~ 2% for 1 hour) to be recorded. We show experimentally that the relaxation of the magnetic moment of the whole sample is reduced strongly if the superconductor is not fully magnetized, either in field cooled (FC) or zero-field cooled (ZFC) conditions. The practical consequence is that an abnormally slow magnetic relaxation (large E-J power law exponent n) under a conventional magnetization procedure indicates that the bulk superconductor could be magnetized further. In a second set of experiments, we study the increase of magnetic moment of a bulk superconductor when it is attached to a soft ferromagnetic disk. We also examine the irreversible demagnetization of these structures when subjected to crossed magnetic fields and compare to the magnetic moment of a classical (Nd-Fe-B) permanent magnets in the same conditions. Acknowledgements: This work is part of an "Action de Recherches Concertees" grant (ARC 11/16 -03) from the "Communaute Francaise de Belgique". [less ▲]

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See detailSuperconductors used as trapped field magnets : investigation of hybrid structures combining superconducting and ferromagnetic materials
Vanderbemden, Philippe ULiege; Fagnard, Jean-François ULiege; Caps, Hervé ULiege et al

Conference (2016, August 30)

In addition to their unique ability to carry electric currents without loss, superconductors are able to trap significant magnetic flux densities at low temperature. In so-called type-II irreversible ... [more ▼]

In addition to their unique ability to carry electric currents without loss, superconductors are able to trap significant magnetic flux densities at low temperature. In so-called type-II irreversible superconductors, these properties are due to the pinning of individual flux lines (vortices) by defects at the nanoscale level. In this talk the influence of pinning on the magnetic properties of bulk superconductors will be explained and illustrated, with the emphasis placed on how pinning can be exploited to produce “trapped field magnets” for practical applications. We will then show how the properties of these trapped field magnets are enhanced when superconductors (SC) are combined to soft ferromagnetic (FM) alloys [1,2]. In particular we investigate the properties of macroscopic FM/SC/FM structures where the superconductor is sandwiched between two ferromagnetic sections, one of each circular face. In particular we show how the properties of the whole structure is influenced by the saturation magnetization and the geometry of the ferromagnetic material. References: 1. M P Philippe, J-F Fagnard, S Kirsch, Z Xu, A R Dennis, Y H Shi, D A Cardwell, B Vanderheyden, and P Vanderbemden, Magnetic characterisation of large grain, bulk Y–Ba–Cu–O superconductor–soft ferromagnetic alloy hybrid structures. Physica C: 502 (2014) 20-30. 2. M P Philippe, M D Ainslie, L Wera, J-F Fagnard, A R Dennis, Y H Shi, D A Cardwell, B Vanderheyden, and P Vanderbemden, Influence of soft ferromagnetic sections on the magnetic flux density profile of a large grain, bulk Y–Ba–Cu–O superconductor. Superconductor Science and Technology: 28 (2015) 095008. [less ▲]

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See detailModel for the decay of the magnetization of a bulk superconductor for different sources of magnetic fields with a transverse component
Vanderheyden, Benoît ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2016, August)

Bulk (RE)BCO samples can be used as efficient trapped-field magnets with surface flux densities of a few teslas. However, their magnetization may decay over time when subjected to time-varying parasitic ... [more ▼]

Bulk (RE)BCO samples can be used as efficient trapped-field magnets with surface flux densities of a few teslas. However, their magnetization may decay over time when subjected to time-varying parasitic crossed fields (applied perpendicular to the initially trapped flux density). In this work, we consider and compare the magnetization decays that occur under different types of crossed-field perturbations, mimicking realistic perturbations that are expected in synchronous motors. The model is based on a finite element approach carried in the GetDP environment. The superconductor is modelled with an isotropic E(J) power law and a field-independent current density. Under a magnetic field perturbation with a non-vanishing transverse component, the superconductor magnetization is modified due to a rearrangement of currents. We first investigate perturbations with different amplitudes that are applied perpendicular to the magnetization axis and have either a bipolar waveform with a zero DC component, or a unipolar waveform with a non-zero DC component. Then, these perturbations are compared to those obtained with an external field applied along the magnetization axis and being rotated away at different angles. The model predicts the evolution over time of the distributions of currents and the net magnetic moment in each case. It is demonstrated that the patterns of reorganized currents strongly depend on the waveform of the applied perturbation and an analysis of these patterns is provided. [less ▲]

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Full Text
See detailComparison of the magnetization decay due to crossed or rotated magnetic field measured on a Ferromagnet disk/GdBCO pellet structure
Fagnard, Jean-François ULiege; Morita, Mitsuru; Teshima, Hidekazu et al

Poster (2016, August)

Superconducting (SC) bulk materials are commonly used as permanent magnets in synchronous rotating machines. When the machine is in operation, the bulk materials that are placed in the machine rotor can ... [more ▼]

Superconducting (SC) bulk materials are commonly used as permanent magnets in synchronous rotating machines. When the machine is in operation, the bulk materials that are placed in the machine rotor can experience a certain misalignment with the magnetic field generated by the stator. This situation is likely to happen during transients or non-ideal behavior of the machine: the angle between the stator field and the SC magnetization can change because of a sudden change of mechanical load or vibrations of the device. The consequence is that the SC pellet can be subjected to a transverse component of magnetic field which is likely to cause a significant demagnetization and therefore a decrease of the machine performances. In previous works, we already have studied the problem of a “pure” transverse magnetic field (also called “crossed field”) as well as the beneficial influence of a ferromagnetic (FM) disk placed on the top of the SC pellet. To this aim, a bespoke insertion tool for a PPMS was designed and constructed. The tool can be used to characterize a SC sample whose orientation is changed from a parallel to a perpendicular direction with respect to the magnetic field direction. Here, this insertion tool was used to rotate the sample at various angles regarding to the field direction. A GdBCO pellet of 9 mm in diameter and 5 mm in height was characterized at 77 K in various experimental conditions by measuring the magnetic induction below the centre of the sample surface. We compared the results obtained with the GdBCO pellet alone to those with a FM disk of various thicknesses attached on the top of the sample. A first set of experiments consisted in applying pure transverse magnetic field cycles of 50 mT and 100 mT at low frequency, the polarity of the field being either always the same (unipolar) or reversed after half of the period (bipolar). A second set of experiments was carried out by rotating the applied magnetic field (applied first in the pellet magnetization direction) in several steps to a given angle (up to 90°) and back to 0°. All these experiments were carried out after having fully magnetized the sample and recorded the magnetization decay due to the flux creep during 30 min. The results show that (i) the FM disk reduces the magnetization decay in both pure transverse and rotated magnetic field experiments, (ii) rotated magnetic fields have a less detrimental effect than the “pure transverse” magnetic field. This phenomenon is likely to be caused by a partial remagnetization of the sample when the angle of the magnetic field is reduced back to 0°. Finite element modelling was carried out to sustain the measurements of the local magnetic induction and gives additional information on the supercurrent distribution inside the SC pellet during “pure transverse” and “rotated” applied fields. [less ▲]

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